Preheat the Oven:

Preheat oven to 350°F (175°C). Grease and flour a 9-inch round or square baking pan.

Prepare the Streusel:

In a small bowl, mix brown sugar, flour, and cinnamon. Cut in the cold butter until the mixture forms coarse crumbs. Stir in nuts if using. Set aside.

Make the Cake Batter:

In a large bowl, cream the butter and both sugars until light and fluffy (2–3 minutes). Add eggs one at a time, then mix in vanilla.

Add Dry Ingredients:

In a separate bowl, whisk together flour, baking powder, baking soda, salt, and cinnamon.

Combine Everything:

Gradually add the dry mixture to the creamed butter, alternating with sour cream. Mix just until combined. Fold in the chopped apples.

Assemble and Bake:

Pour the batter into the prepared pan and spread evenly. Sprinkle the streusel topping over the batter.

Bake:

Bake for 45–50 minutes, or until a toothpick inserted in the center comes out clean. Cool in the pan for at least 15 minutes before slicing.