In a small bowl, whisk together brown and granulated sugars, cinnamon, nutmeg, and salt. Drizzle in melted butter and mix until well combined. Use a spoon to stir in the flour, a little at a time, until flour is completely incorporated. Place in the refrigerator.
In a small bowl, whisk together flour, baking powder, and salt. Set aside.
In a medium bowl, combine butter and granulated sugar. Cream them together until light and fluffy, about 1 minute. Add egg and vanilla and beat until well-combined. Add sour cream and mix well. Add half of the flour mixture and stir until just mixed. Add 1 tablespoon of milk and stir until just mixed. Repeat with the remaining flour mixture and milk.
Bake for 28 to 34 minutes, until a toothpick inserted into the center of the cake comes out with just a few crumbs. Differences in ovens, baking dishes, and environments may affect baking time, so don't be worried if your cake takes slightly longer to bake. Cool for at least 15 minutes before cutting, and enjoy!
*Correct flour measurement is important in this recipe. For best results, measure by weight or the Spoon and Sweep Method (stir the flour in your container to fluff it up and then spoon it into your measuring cup, sweep excess off with the flat of a knife).
This cake is best on the first day, but will keep for up to 48 hours. Store covered, at room temperature.