In a medium bowl, whisk together the flour, salt, and baking powder. Set aside.

In a large bowl, using an electric mixer, cream the butter and sugar until light and fluffy.

Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.

Gradually add the dry ingredients to the wet mixture, alternating with the milk, starting and ending with the flour mixture. Mix until just combined.

Remove about 2 1/2 cups of the batter and mix with the cocoa powder and hot water until smooth.

Spoon the vanilla and chocolate batters alternately into the prepared pan. Use a knife to swirl the batters together, creating a marbled effect.

Bake for 60 minutes, or until a toothpick inserted into the center comes out clean. Allow to cool in the pan for 10 minutes, then turn out onto a wire rack to cool completely.