Rich Marble Pound Cake w/ confetti icing

I was over at a friend's house waiting for our bowling league to start so I decided to bake something. I was flipping through her many cake and dessert books and trying to match what she had available in the fridge for me to use and decided to make this pound cake.

She didn't have a stand mixer but she had a hand held mixer. The directions for the pound cake was to use a stand mixer with the flat beater adjusted on

Ingredients
3 cups cake flour
2 teaspoons baking powder
1/2 teaspoon salt
2 1/2 granulated sugar, divided
1/2 cup natural (not Dutch-processed) cocoa powder
6 tablespoons water
1 1/2 cups (3 sticks) unsalted butter, softened
1 1/2 teaspoons vanilla extract
5 large eggs
1/2 cup whole milk

To make the cake: ~haven't been back to her house so can't complete the instructions at this time~