1. Preheat the oven to 350 degrees F. Lightly grease a 9x13-inch baking pan; set aside.
2. Whisk together the flour, baking powder, salt, cinnamon and nutmeg in a medium bowl; set aside.
3. Beat together the butter and brown sugar on medium speed until light and fluffy, about 5 minutes. Add the eggs one at a time, and then the vanilla. Beat, scraping the bowl, until thoroughly combined. On low speed, gradually add the flour mixture until just combined. Give the dough a final stir with a spatula or wooden spoon to make sure the flour is incorporated.
4. Spread the dough evenly into the pan (I found an offset spatula was the best tool for the job, as it's a thick batter). Combine the granulated sugar and cinnamon in a small bowl and sprinkle evenly over the top of the batter.
5. Bake for 25-30 minutes or until the surface springs back when gently pressed. Cool completely before cutting. Store in an airtight container at room temperature.