Preheat your oven to 350°F (175°C). Thoroughly grease and flour a 9-inch round cake pan to prevent sticking.

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

In a large mixing bowl, cream the softened butter and sugar together until light and fluffy, about 3-4 minutes using an electric mixer.

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

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

Transfer about one-third of the batter to a separate bowl. In a small dish, mix cocoa powder with hot water until smooth, then fold this chocolate mixture into the separated batter until fully incorporated.

Spoon batters alternately into the prepared pan, creating a checkerboard pattern. For the classic marbled effect, use a knife or skewer to gently swirl the batters together with a figure-eight motion. Don't overmix - you want distinct swirls of vanilla and chocolate.

Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted into the center comes out clean with a few moist crumbs.

Allow the cake to cool in the pan for 10 minutes, then carefully turn out onto a wire rack to cool completely before slicing and serving.