In a medium bowl, combine brown sugar, flour, cinnamon, and chopped pecans. Cut in cold butter until the mixture resembles coarse crumbs. Chill while you prepare the cake.

Set your oven to 350°F (175°C). Grease a 9×13-inch pan with butter or line it with parchment paper.

In a large bowl, cream the softened butter and granulated sugar together until fluffy. Add the eggs one at a time, mixing well after each addition. Stir in the sour cream and vanilla extract.

In another bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, and nutmeg. Gradually stir this mixture into the wet ingredients until just combined.

Spread half of the batter into the prepared pan. Arrange the sliced Granny Smith apples evenly over the batter.

Dollop the remaining batter over the apples and smooth it out with a spatula.

Sprinkle the streusel topping evenly over the batter and lightly press it down.

Bake for 45–55 minutes, or until a toothpick inserted in the center comes out clean.

Allow the cake to cool in the pan for about 20 minutes before serving. Drizzle with caramel sauce if desired.