Apples lend their sweet-tart flavor to this warm and comforting apple coffee cake. The nutty oat and pecan filling in the middle of the cake also acts as a crispy topping that's drizzled with a sweet vanilla glaze.
Preheat oven to 350 degrees F. Coat a 9-inch-square baking dish (at least 2 1/4 inches deep) with baking spray.
Combine 3/4 cup brown sugar, 1/2 cup butter and granulated sugar in the bowl of a stand mixer fitted with a paddle attachment. Beat on medium speed until light and fluffy, about 3 minutes. Add eggs, 1 at a time, beating on low speed just until blended after each addition. Add 2 teaspoons vanilla; beat on low until blended.
Whisk 1 3/4 cups flour and 2 teaspoons cinnamon in a medium bowl. Add baking soda, baking powder and salt; whisk to combine. Add the flour mixture to the butter mixture alternately with yogurt, beginning and ending with the flour mixture and beating on low speed just until blended after each addition. Fold in apples.
Combine oats, pecans and the remaining 1/2 cup flour, 1/2 cup brown sugar and 1 teaspoon cinnamon in a medium bowl. Add the remaining 1/4 cup butter and use your fingers to work it into the mixture until blended and crumbly.
Spread half the batter in an even layer in the prepared baking dish. Sprinkle evenly with half the oat-pecan mixture. Spoon the remaining batter on top and spread it to the edges of the pan. Top with the remaining oat-pecan mixture.
Bake until a wooden pick inserted in the center of the cake comes out clean, 45 to 55 minutes. If necessary, loosely cover with foil after about 30 minutes to prevent excessive browning. Let cool in the pan on a wire rack for 15 minutes.
Whisk confectioners' sugar, milk and the remaining 1 teaspoon vanilla in a small bowl until smooth. Drizzle over the warm cake. Serve warm, or cool completely, about 45 minutes. Cut into 16 squares.
To make ahead: Cover and store at room temperature for up to 1 day.
This was delicious with a cup of milky coffee (also great with hot tea!).