Making the marbré cake batter:
Preheat the oven to 375°F.
Separate the egg yolks from the whites.
In a mixing bowl, cream 5 tablespoons butter with ½ cup granulated sugar using an electric mixer. Once creamy, add one egg yolk at a time and beat well. Add the vanilla paste (or vanilla extract).
In a separate bowl, combine the flour and baking powder. Sift the dry ingredients.
Add the dry ingredients and the milk gradually, alternating then until the batter is smooth.
In another bowl, place the 3 egg whites. Add salt. Beat the egg whites for about 2 minutes at medium speed. Increase to maximum speed and keep beating for another 2 minutes until the whites are stiff. Do not overbeat or the texture will become grainy.
Pour ⅓ of the egg white mixture into the cake batter mixture and gently stir everything to soften the batter. Pour in the rest of the egg whites and, using a wooden spoon, gently fold the egg whites into the batter to get an airy batter.
Divide the batter in half, transferring the other half in another mixing bowl.
Make a "double boiler" (a pot filled with hot water, covered with a piece of cloth) and place a stainless steel bowl filled with milk chocolate chips on top. Slowly melt, stirring frequently. Add the instant coffee (if using) and molasses. Turn off the heat. Stir well until the mixture is uniform. Mix the chocolate in one of the bowls and using a spatula, stir until the color is uniform.
Grease a cake pan with butter. Dust with flour, then line with parchment paper.
Start by pouring in 1 layer of vanilla cake batter. Top with a layer of chocolate batter. Alternate the layers until all the batter is used, finishing with the vanilla-flavored one.
Bake the cake for 20 minutes at 375°F, then lower the temperature to 350°F and bake for another 40-45 minutes. A skewer or a toothpick inserted into the center of the cake should come out clean.
Remove from the oven. Allow to cool completely before slicing.