Step 1

Preheat oven to 350 degrees and butter a 9-inch square pan or line it with parchment paper.

Step 2

In a medium bowl, whisk together the 2 cups of flour, baking powder, baking soda, and salt.

Step 3

In a mixing bowl, cream the butter. Add both sugars and beat until light and fluffy, about 3 minutes.

Step 4

Add egg and vanilla and mix until combined.

Step 5

Add half the dry ingredients and mix until just barely combined. Add half the sour cream and mix until barely combined. Add remaining flour, mix, and then remaining sour cream. Mix until smooth.

Step 6

Pour batter into the prepared cake pan.

Step 7

Sprinkle the blueberries evenly over the top. Gently press them down into the batter so that they're slightly embedded.

Step 8

in a medium bowl, mix together the flour and both sugars. Add the chilled butter and with a pastry cutter or two forks, mix until a coarse meal is formed.

Step 9

Sprinkle the crumb topping over the blueberries.

Step 10

Bake 40-50 minutes or until a toothpick inserted into the middle of the cake comes out clean. Cool before serving.