Lemon Blueberry Crumb Cake

Place blueberries, 3 tablespoons white sugar, and 1 tablespoon lemon juice into a saucepan over medium heat. Cook, stirring frequently, until blueberries begin to release their juices. Mash blueberries with a potato masher until broken up. Bring mixture to a simmer, and reduce heat to low.

Mix together cornstarch and water in a small bowl until combined. Add to blueberry mixture, stirring constantly. Cook until mixture has thickened, 1 to 2 minutes. Pour blueberry filling into a shallow bowl; place in the refrigerator to cool while you prepare the rest of the cake.

For the crumb topping, combine 1/2 cup flour, brown sugar, 1/8 teaspoon salt, and 1/8 teaspoon nutmeg in the bowl of a food processor. Pulse a few times to combine. Add in 1/4 cup cold butter, and pulse several more times until mixture resembles coarse sand. Pour mixture into a small bowl. Using your hands, press mixture together until it forms medium to large-sized crumbs. Place crumbs into the refrigerator while you prepare the cake.

Preheat the oven to 350 degrees F (175 degrees C). Line an 8×8-inch square pan with enough parchment paper to have overhang on all sides.

To the bowl of the food processor (no need to clean it after preparing the crumb topping), add 1 1/2 cups flour, 1/2 cup white sugar, baking powder, baking soda, 1/2 teaspoon salt, and 1/4 teaspoon ground nutmeg. Pulse a few times to combine. Add in 1/2 cup cold butter, and pulse several more times until mixture resembles coarse crumbs.

In a small bowl or glass measuring cup, mix together buttermilk, egg, lemon zest, lemon juice, and vanilla until combined. Add this mixture to the food processor, and pulse until mixture just comes together, and no dry bits of flour remain, stopping to scrape down the sides of the bowl as necessary. Batter will look somewhat lumpy from the cold butter, but this is normal.

Pour about 2/3 of the batter into the prepared pan, and smooth into an even layer. Remove blueberry filling from the refrigerator and spread evenly over the batter in the pan. Place dollops of remaining batter over the blueberry filling, and spread until most of the filling is covered with a thin layer of batter. (It is ok if not all the blueberry filling is covered with batter.) Remove crumb topping from the refrigerator and sprinkle evenly over the top of the batter. Very lightly press crumbs down to slightly adhere to the batter.

Bake in the preheated oven for 45 to 55 minutes, or until the middle of the cake springs back lightly when touched. Cool cake in the pan until nearly room temperature, about 10 minutes. Remove cake, using the parchment overhang, to a wire rack to cool completely, about 20 minutes. Cut into 12 pieces.