Zebra Bundt Cake

cake flour2 teaspoons baking powder½ teaspoon salt2 ½ cups granulated sugar, divided½ cup natural (not Dutch-processed) cocoa powder6 tablespoons water1 ½ cups unsalted butter, melted and cooled1 ½ teaspoons vanilla extract5 large eggs½ cup whole milk

bittersweet chocolate⅔ cup heavy cream