Step 1

Preheat oven to 350 degrees. Grease a 9x13 inch baking pan and set aside.

Step 2

In a medium bowl whisk together flour, brown sugar, and cinnamon.

Step 3

Cut chilled butter cubes into the flour mixture until mixture resembles coarse crumbs. Cover and chill until ready to use.

Step 4

Toss apples with 2 tablespoons flour to coat and set aside.

Step 5

In a large bowl cream together the butter and sugars and light and fluffy.

Step 6

Mix in the eggs, vanilla and sour cream.

Step 7

Whisk together the 3 2/3 cup flour, salt, and baking powder.

Step 8

Add half of flour mixture to the wet ingredients and mix until just combined. Mix in half of milk, then remaining flour mixture, followed by remaining milk (being careful not to over-mix at any point).

Step 9

Gently stir apples into cake batter.

Step 10

Carefully spread the cake batter into the prepared pan.

Step 11

Sprinkle streusel mixture over the cake batter.

Step 12

Bake in preheated oven for 40-45 minutes until a toothpick inserted in the middle comes out clean. Remove from the oven and allow to cool until just warm.

Step 13

Whisk together powdered sugar, milk, and vanilla for the glaze. Drizzle over cooled coffee cake and serve.