Preheat oven to 350°F (175°C) and grease a 9-inch cake pan.
In a bowl, whisk together flour, baking powder, baking soda, cinnamon, nutmeg, and salt.
In another bowl, cream butter and sugar together until fluffy.
Beat in eggs, then stir in pumpkin puree and vanilla.
Add the dry ingredients alternately with buttermilk until smooth.
Pour batter into prepared pan.
For the topping, mix brown sugar and cinnamon, then sprinkle over the batter.
Bake for 40 minutes, or until a toothpick comes out clean.