Preheat the oven to 350 degrees F and grease a 9x13 inch baking pan.
Cream the butter and sugar together in an electric mixer until light and fluffy. Add the eggs and vanilla and mix well.
Add the flour, baking powder, and salt and mix until just combined.
Mix in the walnuts and then spread the batter into the bottom of the greased pan.
Bake for 18-21 minutes until toothpick comes out clean. Cool completely before frosting.