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1
Berry Sauce: Bring the blueberries, sugar, water and the salt to a simmer in a medium saucepan over medium heat, stirring occasionally. Cook until the sugar is dissolved and the fruit is heated through, 2 - 4 minutes.
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2
Transfer the mixture to a blender and puree until smooth, about 20 seconds. Strain the puree through a fine mesh strainer, pressing on the solids to extract as much puree as possible (you should have about 1/2 cup). Spoon the sauce evenly into 6 - 4 ounce ramekins and refrigerate until chilled, about 20 minutes.
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3
Mouse: Pour the water into a small bowl, sprinkle the gelatin evenly over the tops and set aside to let the gelatin hydrate for 10 minutes. In a separate bowl, whisk the yogurt, heavy cream, lemon zest, lemon juice, vanilla and salt together until smooth.
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4
Whisk the egg whites, sugar and cream of tartar together in a large bowl (or the bowl of a stand mixer) Set the bowl over a large saucepan of barely simmering water, making sure the water does not touch the bottom of the bowl. Heat the mixture, whisking constantly, until it has trippeled in size and registers about 160 degrees on an instant read thermometer, 5 - 10 minutes.
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5
Protecting the hands, take the bowl off the heat, quickly whisk in the hydrated gelatin until melted. Whip the warm mixture with an electric mixer on medium high speed until it forms stiff, shiny peaks, 4 - 6 minutes. Add the yogurt mixture and continue to whip until just combined, 30 - 60 seconds.
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6
Divide the mousse evenly amoung the chilled ramekins containing the blueberry sauce, cover tightly with plastic wrap and chill until set, 6 - 8 hours. Serve chilled.
