Preheat oven to 325.
Beat butter and one and a half cups sugar at medium speed with a stand mixer for 4 to 5 minutes or until creamy.
Add eggs one at a time, beating until just blended after each addition.
Beat in vanilla extract.
Sift together flour, baking powder and salt.
Add to butter mixture alternating with half and half, beginning and ending with flour mixture.
Beat at low speed just until blended after each addition. Stop to scrape bowl as needed.
Spoon 1 1/4 cups batter into a 2 qt. bowl and stir in cocoa, 3 tablespoons of hot water and remaining 1/4 cup sugar. Stir until well blended.
Spread remaining vanilla batter into a greased and floured 15 x 10 inch jelly roll pan.
Spoon chocolate mixture over vanilla and gently swirl with a knife.
Bake at 325 for 23-28 minutes or until a wooden toothpick inserted into the center comes out clean.
Cool completely in pan on the wire rack for about an hour.
Frost with Mocha Frosting.