• Preheat oven to 350° and spray an 8 x 8 baking dish with nonstick spray.

For The Streusel Topping and Apple Streusel Filling

• Whisk brown sugar, sugar, cinnamon, and pecans together. Reserve ¾ cup for the filling.
• Add flour, butter, and vanilla to the remaining mixture and form a crumble streusel topping.

For The Coffee Cake

• In a small bowl, mix egg yolks and about ⅓ cup of the sour cream and vanilla. Set aside.
• In a large bowl, whisk flour, sugar, baking powder, baking soda, and salt.
• Add butter and remaining sour cream. Mix until well combined.
• Gradually add egg mixture, a little at a time, into dry ingredients, mixing well. Batter will be thick and lumpy.
• Add ⅔ of the batter into prepared baking dish and smooth out the batter.
• Sprinkle streusel filling over the top of batter, followed by chopped apples.
• Add remaining batter on top of the apples.
• Top with remaining streusel and bake for 55-60 minutes or until a toothpick inserted comes out clean. Set on cooling rack.