Preheat oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.
In a large bowl, cream together the softened butter and sugar until light and fluffy.
Beat in the eggs one at a time, then stir in the vanilla extract.
In a separate bowl, whisk together the flour, cinnamon, nutmeg, salt, and baking powder.
Gradually add the dry ingredients to the wet ingredients, alternating with the milk, beginning and ending with the dry ingredients. Mix until just combined.
Stir in the mashed sweet potatoes.
Pour batter into the prepared pan and bake for 50-60 minutes, or until a wooden skewer inserted into the center comes out clean.
Let cool completely before serving.
Notes
For a richer flavor, use brown sugar instead of granulated sugar. You can also add chopped nuts or chocolate chips to the batter.