Preheat oven to 350 degrees.

Grease a 10 inch square pan with butter. In a medium bowl, combine the flour, brown sugar, cinnamon, nutmeg and butter. Stir with a fork until clumps form. Set aside.

In a large mixing bowl, beat the butter and the sugar together until light and fluffy, about 3 to 4 minutes. Stir in the eggs and vanilla extract. Add the flour, baking powder and cinnamon. Stir to combine all the ingredients. Mix in the sour cream.

Pour half of the batter into the prepared pan. Use a spatula to spread a thin layer across the bottom of the pan. Sprinkle half the cinnamon streusel over the batter and spoon the remaining batter over the streusel layer. Sprinkle the remaining streusel over the batter in the pan.

Bake for 40 to 42 minutes, until lightly browned and a toothpick comes out with moist crumbs. Let cool completely before slicing.