In a medium bowl, whisk flour, sugar, brown sugar and salt together.
Work in butter with your fingertips until the mixture looks like the consistency of wet sand. Press together to make chunks. Set aside in the refrigerator until ready to use.
Crumb Cakes
Preheat oven to 350ºF (180ºC). Line a 6-serving shallow jumbo muffin pan* with jumbo muffin liners.** Jumbo muffin liners will fit by pressing them around the inside bottom edges.
In a medium bowl, whisk together the flour, baking powder and salt.
In a large bowl, cream the butter, sugar and lemon zest with an electric mixer on medium speed, until light and fluffy.
Beat the egg, egg yolk and extracts into the butter mixture in 3 separate additions, making sure each addition is incorporated before adding the next.
In a small bowl, whisk the milk and sour cream together.
On low speed, mix the flour mixture into the batter in two additions alternating with half of the milk mixture after each addition. Beat just until combined.
Transfer batter to the prepared baking pan, evenly distributing the batter and filling the cups about half full.
In a medium bowl, toss the blueberries with 1 teaspoon of flour and place them into the cups on top of the batter, about 1 ounce (28g) in each. Top each one with crumb topping.
Bake until a toothpick inserted into the center of the cakes comes out clean, about 25-30 minutes. Let cakes cool in the pan for 5 minutes, then transfer to a cooling rack to cool completely.
Cream Cheese Icing
In a medium bowl, whisk together the cream cheese, melted butter, powdered sugar, salt and 1 tablespoon of lemon juice. Add additional lemon juice, a little at a time until the desired consistency is reached. Drizzle over the cooled cakes.