Preheat your oven to 350°F (175°C) and grease a 10-inch square baking pan with softened butter.

In a medium bowl, combine the all-purpose flour, brown sugar, ground cinnamon, and nutmeg. Stir in melted butter until crumbly for the streusel topping.

In a large bowl, beat softened butter and granulated sugar until light and fluffy. Add eggs one at a time, then add vanilla extract.

Gradually add all-purpose flour, baking powder, and cinnamon to the mixture. Stir until just combined, then fold in sour cream.

Spread half of the cake batter into the prepared pan. Sprinkle half of the streusel topping over it.

Spread the remaining batter over the streusel, then top with the remaining streusel.

Bake in the preheated oven for 40-42 minutes, or until the top is lightly browned and a toothpick comes out with moist crumbs.

Allow the cake to cool in the pan on a wire rack for 30 minutes before cutting into squares to serve.