Preheat the oven to 350°F. Lightly spray a 9x13 baking pan with nonstick cooking spray.
In a bowl with a hand mixer or in a stand mixer, cream together the butter and sugar. Add the eggs and mix well.
In another bowl, whisk together the flour, baking powder, and salt. Add 1/2 of the dry mixture to the butter mixture and mix well. Add 1/2 of the milk and mix well. Add the other half of the dry mixture and mix well. Add the other half of the milk and mix well. Add the vanilla and mix well.
Pour the batter into the prepared pan and spread it evenly. Bake for 25 t0 30 minutes or until set and the cake starts to pull away from the sides. You can also check the doneness by inserting a toothpick into the center of the cake. If the toothpick comes out clean, the cake is done. Cool completely before icing.