recipe from Noble Pig blog
Brown-Sugar Coffee Cake
1-1/2 cups all-purpose flour
1/2 cup granulated sugar
2 teaspoons baking powder
1/2 teaspoon salt
1 egg, large
1/2 cup sour cream
1/4 cup whole milk
4 Tablespoons unsalted butter, melted and slightly cooled
1 cup Granny Smith Apple, peeled, cored and chopped
1/2 cup packed light brown sugar
1/4 cup all-purpose flour
3 Tablespoons unsalted butter, cold, cut into 1/2" pieces
1/2 cup chopped walnuts
In a large bowl, whisk together the flour, granulated sugar, baking powder and salt.
In a medium bowl, whisk the egg just until the yolk and white are blended. Add the sour cream, milk and melted butter and mix well. Pour the egg mixture into the flour mixture and stir just until the ingredients are blended. Stir in the chopped apple.
Spread the batter in a buttered 8 x 8 square baking pan. To make the topping, in a bowl, combine the brown sugar, flour and butter. Using your fingertips, work the ingredients together until a coarse, crumbly mixture forms. Add the walnuts and mix well with your fingertips. Sprinkle the topping over the batter.
Bake the coffee cake at 400 degrees for 25-30 minutes until the topping is browned and a sharp knife inserted into the center comes out clean. Let cool for about 5 minutes, then cut into squares and serve warm or at room temperature.