1. Allow butter and eggs to stand at room temperature for 30 minutes. Grease a 9x13x2 inch pan. Set aside. In a medium bowl, stir together flour, cocoa powder, baking soda and salt. Set aside
2. Preheat oven to 350°. In large mixing bowl, beat butter with an electric mixer for 30 seconds. Gradually add sugar, 1/4 c at a time, beating on medium speed until well combined. Scrape sides of bowl; beat for 2 minutes. Add eggs, one at a time, beating after each addition. Beat in vanilla. Alternately add flour mixture and milk to butter mixture, beating on low speed after each addition just until combined. Beat on high speed for 20 seconds more. Spread into prepared pan.
3. Bake for 35-40 minutes until toothpick inserted near center comes out clean. Cool thoroughly on wire rack. Frost if desired with your choice of frosting. I recommend Chocolate Ganache.