Step 1: Preheat your oven to 350°F (175°C) and grease a 9x9 inch square baking pan to prepare for the apple and cinnamon streusel coffee cake batter.
Step 2: In a medium bowl, whisk together the flour, baking powder, baking soda, and salt for the base of the apple and cinnamon streusel coffee cake.
Step 3: In a large bowl, cream the softened butter and granulated sugar together until light and fluffy, usually about 3 minutes.
Step 4: Add the eggs one at a time to the butter mixture, beating well after each addition, then stir in the vanilla extract.
Step 5: Gradually add the dry ingredients to the wet ingredients, alternating with the sour cream, beginning and ending with the flour mixture to ensure a smooth apple and cinnamon streusel coffee cake batter.
Step 6: To make the streusel, combine the brown sugar, flour, and cinnamon in a small bowl. Cut in the cold butter with a fork or pastry blender until the mixture resembles coarse crumbs.
Step 7: Spread half of the cake batter into the prepared pan, top with the chopped apples, and then carefully spread the remaining batter over the fruit.
Step 8: Sprinkle the streusel topping evenly over the top of the apple and cinnamon streusel coffee cake.
Step 9: Bake for 50-60 minutes, or until a toothpick inserted into the center of the apple and cinnamon streusel coffee cake comes out clean.
Step 10: Allow the apple and cinnamon streusel coffee cake to cool in the pan for at least 20 minutes before slicing and serving.