Preheat your oven to 350 °F. Grease a 9x9 pan and set aside.
In a medium bowl, whisk together the flour, baking powder and soda, salt and nutmeg. Set aside.
In a large bowl cream together the butter and sugar until light and fluffy. Beat in the eggs, one at a time, until fully combined, then mix in the vanilla.
Add the dry ingredients alternately with the sour cream to the butter mixture. Mix until smooth.
Spread the batter into the prepared pan. Tap a few times on the counter to remove any air bubbles.
Combine the topping ingredients in a small bowl, working the butter into the dry ingredients until small pieces, then sprinkle in an even layer on top of the batter,
Bake for 30-35 minutes, or until a toothpick or cake tester inserted comes out clean. Do not overbake or it will dry out!
Serve warm immediately for the best cake. Store for up to 24 hours in a closed container at room temperature, coffee cake tends to dry out fast. Do not refrigerate.