This coffee cake is best served warm. To prepare ahead, cool completely, wrap (still in the pan) in foil, and leave out at room temperature for up to one day, or freeze for up to three months. To reheat thawed cake, unwrap and bake at 250° for 15 to 20 minutes.
1 1/2 cups all-purpose flour (about 6 3/4 ounces)
1 cup granulated sugar
1 1/2 teaspoons baking powder
1 1/2 teaspoons ground cinnamon
1/2 teaspoon salt
3/4 cup 1% low-fat milk
2 tablespoons butter, melted
1 teaspoon vanilla extract
1 large egg, lightly beaten
1 cup diced peeled Granny Smith apple (about 1 apple)
1/4 cup packed brown sugar
2 tablespoons all-purpose flour
1/2 teaspoon ground cinnamon
2 tablespoons chilled butter, cut into small pieces
How to Make It
Preheat oven to 350°.
To prepare cake, lightly spoon 1 1/2 cups flour into dry measuring cups; level with a knife. Combine flour and next 4 ingredients (through salt) in a large bowl, stirring with a whisk. Make a well in center of mixture. Combine milk, melted butter, vanilla, and egg, stirring with a whisk; add to flour mixture, stirring just until moist. Fold in apple. Pour batter into an 8-inch square baking pan coated with cooking spray.
To prepare streusel, combine brown sugar, 2 tablespoons flour, and 1/2 teaspoon cinnamon; cut in butter with a pastry blender or 2 knives until mixture resembles coarse meal. Sprinkle streusel evenly over batter. Bake at 350° for 45 minutes or until a wooden pick inserted in center comes out clean. Cool in pan 10 minutes on a wire rack before serving. Serve warm.