Place the butter, oats and cocoa in a large bowl; add boiling water and stir until butter is melted. Stir in brown sugar and vanilla. Add eggs, one at a time, beating well after each addition. Combine the flour, baking powder, baking soda and salt; gradually add to oat mixture just until combined.
Pour into a greased 13x9-in. baking pan. Bake at 375° for 30-35 minutes or until a toothpick inserted in the center comes out clean. Cool completely on a wire rack.