Preheat your oven to 350 degrees, and line an 8x8 inch pan with parchment paper. Lightly spray the pan with cooking spray. Set aside.

In a small bowl, whisk together the boiling water and cocoa powder until a smooth, thick paste is formed. Set aside.

In a large bowl, beat together the butter, oil, and sugar until fluffy. Add in the eggs one at a time, mixing between each addition. Beat in the mayonnaise and vanilla.

In a medium bowl, stir together the flour, baking powder, and salt. Mix in about half of the dry mixture, and then add mix in the milk. Finish by stirring in the remaining flour mixture until well combined- the mixture will be thick.

Pour about 1/3 of the batter into a medium bowl- it does not have to be exact. Beat in the cocoa powder-water mixture to the 1/3 batter until completely combined.

Scoop the vanilla batter into your pan with an ice cream scoop, leaving spaces for the chocolate batter. Then, scoop the chocolate batter onto the pan as well. Using a spatula, spread the batter to the edges of the pan. Then, use a butter knife to swirl the batters together.

Bake for 30-35 minutes, or until a cake tester comes out clean. Cool completely before frosting.

Beat together the butter, powdered sugar, salt, and vanilla until smooth. Beat in the melted chocolate, and then the heavy cream until the mixture is fluffy. Spread evenly onto the cooled cake. Enjoy!