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To make the crust:
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Add the butter in a medium bowl, beat with an electric mixer on medium-high until smooth.
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Beat in the sugar and salt until well blended.
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Scrape down the sides of the bowl if needed, then beat in the egg yolk.
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Beat in half of the flour until just crumbly.
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Scrape down the bowl, beat in remaining flour and then the yogurt or sour cream, beating until the dough is just incorporated.
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Transfer the dough onto a lightly floured working surface or a cutting board and knead lightly to bring it together.
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Roll the dough between 2 generously floured sheets of waxed paper into a circle about 1/4 inch thick with a rolling pin.
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Refrigerate for 1 hour.
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Cut out 6 (4 1/2-inch) rounds using an inverted bowl or round cookie cutter.
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Arrange rounds in the muffin tins and, using a small shot glass or your fingers, press into the corners and about halfway up the sides for a snug fit.
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Freeze dough in the muffin tin for half an hour.
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Put a rack in the lower third of the oven and preheat to 325F (160C).
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Coat the outside of 6 standard muffin liners with cooking spray and transfer in the crusts.
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Fill with dried beans or pie weights.
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Bake until start to brown around the edges, about 25 to 30 minutes.
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Remove from the oven and cool for about 2 minutes, then carefully remove the muffin liners and baking beans or pie weight.
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Bring the pan back to the oven and keep baking until crusts are cooked through and evenly browned, about 18 to 22 minutes more.
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Cool for a few minutes in the muffin tins on a wire rack.
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Carefully remove crusts from the muffin tin and let cool completely on a rack.
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To make the Lemon Filling:
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Add the sugar, cornstarch, and yolks in a saucepan over medium-low heat.
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Whisk frequently until the mixture is smooth and sugar dissolves, 1 to 2 minutes.
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Stir in the key lime juice and keep cooking, whisking frequently, until the mixture is as thick as sour cream and is just about to simmer, about 4 minutes.
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(Make sure to stir into the sides of the pot so that all curd thickens evenly.)
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Strain through a fine mesh strainer into a bowl.
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Whisk in the butter a little at a time, until smooth.
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Stir often until cool to room temperature.
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(Setting the bowl in a larger bowl of ice will speed the process up.
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).
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Fill each mini tart with about 4 1/2 tablespoons of the cool lime filling.
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To make the Meringue:
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Bring a few inches of water to a boil in a saucepan that can hold a standing mixer's bowl above the water.
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Use your hand to whisk together the egg whites, sugar, cream of tartar and salt in the bowl.
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Place the bowl above the boiling water and keep whisking until the mixture is hot to the touch (135 degrees F) and the sugar dissolves, about 2 minutes.
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Transfer the bowl to the mixer fitted with the whisk attachment and beat the whites at medium-high speed until they hold soft peaks.
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Increase speed to high and go on beating to make a soft and glossy meringue, about 2 minutes.
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Dollop or pipe on top of the filling.
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Just before serving, preheat the broiler to high.
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Set the tartlets on a baking sheet, and place under the broiler until the meringue is evenly baked, about 2 minutes.
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(Alternatively, brown meringue with a blowtorch.)
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Serve right away or keep refrigerate until ready to serve.
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Notes:
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They can be stored in the refrigerator for up to 2 days.
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Or they can be frozen very well.
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They can be prepared, assembled and frozen up to 1 week in advance.
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Defrost for 20 minutes before broiling the tops and serving.
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The crusts can be made, baked and frozen up to 2 weeks.
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The curd can be made up to 3 days ahead and keep refrigerate.
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The meringue can be made up to 1 day ahead and keep refrigerate.
