Preheat oven to 325 degrees. Lightly grease and flour an 9x13 metal pan.
In a medium bowl, combine flour, baking powder, baking soda, salt, cinnamon and nutmeg. Set aside.
In a liquid measuring cup, combine buttermilk and vanilla. Set aside.
In a large bowl, beat sugar, butter and oil together until creamed and combined. Add eggs, one at a time, making sure to incorporate after each egg. Beat until light and fluffy.
On low speed, add the flour mixture in 3 additions, alternating with 3 additions of the buttermilk mixture. Beat until smooth.
Pour batter into a greased and floured 10x15 pan and smooth the top with a spatula. Bake at 325, for 26-32 minutes, or until golden brown (or when a toothpick inserted in the middle comes out clean).
While still warm, poke holes in the cake with a skewer about ½ - 1 inch deep.
For the glaze, add melted butter to a large bowl, along with sugars, milk and vanilla. Beat on medium speed until combined and smooth and quickly drizzle over warm cake. The glaze sets up fast, so be sure to spread the glaze as soon as possible.