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1
Preheat oven to 325. Grease an 8-inch-square baking pan, and/or line the pan with parchment paper if you'd like to remove the finished coffee cake from the pan before slicing.
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2
Prepare the fruit filling in a small mixing bowl by tossing the berries with the first 3 tablespoons of granulated sugar, the cornstarch, and the zest of the first 1/2 lemon. Set the bowl aside while you make the crumb topping and cake batter.
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3
Prepare crumb topping by first combining the melted butter with the sugars in a large mixing bowl, stirring until smooth. Add the flour gradually, stirring together with a wooden spoon. Smush the resulting stiff dough-like mixture together in the bottom of the bowl. Set aside.
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4
Next, whisk the Greek yogurt, egg & yolk, vanilla extract, and zest of the second 1/2 of the lemon together in small-ish bowl.
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5
In the bowl of a stand mixer fitted with the paddle attachment, combine the flour, sugar, baking soda, baking powder, and salt.
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6
Add the butter and a spoonful or two of the yogurt/egg mixture, and mix on medium low speed. Once the flour is safely moistened, raise speed to medium and beat for 30 seconds.
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7
Add 1/2 of the remaining yogurt/egg mixture and beat for 20 seconds. Scrape down the sides of the bowl and repeat with the last bit of the mixture, making sure the batter is well combined.
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8
Spread a little more than 1/2 of the batter in the bottom of the pan, and then top evenly with the berries. Be sure to include any juices hanging out in the bottom of the bowl.
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9
Apply the remaining batter to the top in small dollops, and carefully spread it around to cover the top. This does not need to be in any way perfect.
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10
Break your topping mixture into crumbs and distribute evenly across the top, trying to keep them in mostly big chunks (about 1/2 inch) but including all the little crumbies as well.
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11
Pop in the preheated oven and bake for 40-50 minutes. The filling will bubble through the crumbs in places, but have no fear! That's part of the cake's rustic charm. The cake is done when a toothpick inserted in the center comes out cleanly.
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12
Allow cake to cool before cutting and serving. It's delicious served for breakfast or accompanied by lemon ice cream for a summery dessert. It also holds up well in the fridge for about a week (if it manages to last that long without being eaten).
