This apple coffee cake, made moist with yogurt, features a fruity layer of Granny Smiths and buttery streusel in the middle, plus more streusel on top.
Preheat the oven to 350 degrees F (175 degrees C). Grease and flour a 9- or 10-inch Bundt pan.
Combine brown sugar, ¾ cup flour, and cinnamon in a bowl; cut in ½ cup cold butter with a fork until crumbly. Stir in walnuts. Set streusel aside.
Whisk 3¼ cups flour, baking powder, and baking soda together in a bowl. Set aside.
Beat white sugar and ¾ cup butter together in a large bowl using an electric mixer until light and fluffy; beat in eggs, one at a time, beating well after each addition. Stir in yogurt and vanilla extract; stir in flour mixture until just blended.
Pour 3 cups batter into the prepared Bundt pan; sprinkle with ¼ streusel then layer with apples. Sprinkle with ½ remaining streusel. Pour in remaining batter; top with remaining streusel and lightly pat down into batter.
Bake in the preheated oven until a toothpick inserted into center comes out clean, 50 to 60 minutes. Cool in the pan on a wire rack for 15 minutes. Tap the pan firmly a few times, then shake gently to loosen the cake. Place a cookie sheet over the pan and carefully invert to release cake. Remove the pan; transfer cake to the rack and cool completely.