01 -
Preheat the oven to 350°F (175°C). Grease and flour a 9-inch square baking pan.
02 -
In a medium bowl, whisk together all-purpose flour, baking powder, baking soda, and salt.
03 -
In a large bowl, cream the softened butter and granulated sugar until light and fluffy.
04 -
Beat in the eggs one at a time, followed by the vanilla extract.
05 -
Alternately add the dry ingredients and sour cream to the butter mixture, beginning and ending with the dry ingredients. Mix until just combined.
06 -
Pour half of the batter into the prepared pan, spreading evenly. Spoon apple pie filling over the batter, then sprinkle with chopped walnuts if desired.
07 -
Spread the remaining batter carefully over the apple pie filling to cover it completely.
08 -
In a small bowl, combine brown sugar, all-purpose flour, cubed cold butter, and ground cinnamon using a fork until crumbly.
09 -
Sprinkle the crumb mixture evenly over the top of the batter layer.
10 -
Bake for 45 to 50 minutes, or until a toothpick inserted in the center comes out clean.
11 -
Allow the coffee cake to cool in the pan for 15 minutes before transferring to a wire rack to cool completely.