For the cupcakes: Preheat oven to 350°F (175°C). Line muffin tins with 14 cupcake liners.

Spoon 1 teaspoon of the vanilla batter into each cupcake liner, followed by 1 teaspoon of the chocolate batter, alternating until the batters are used up. Gently swirl the batters together with a knife to create the marble effect. Bake for 16-18 minutes until a toothpick comes out clean. Remove from oven and cool in the pans on a wire rack for 5 minutes. Transfer cupcakes to a wire rack to cool completely.

For the frosting: Beat the butter in the bowl of a stand mixer on high speed until light and fluffy. Add the powdered sugar, one third at a time, and beat on high speed for several minutes until mixture is light and creamy. Beat in vanilla. Add the cream cheese and beat on high for a couple of seconds to incorporate. Scrape the bowl and beat on high again for a couple of seconds. Transfer half of the frosting (about 300 g) to another bowl. Beat the cooled melted chocolate into the remaining frosting to make the chocolate frosting.

To frost the cupcakes: For marbled frosting, use a large star tip and fill one side of the piping bag with vanilla cream cheese frosting and fill the other half with the chocolate cream cheese frosting so the two frostings are side by side in the piping bag (see photo). Pipe swirls of frosting onto the cooled cupcakes.

Store frosted cupcakes tightly covered in the refrigerator for up to four days. The cupcakes taste best if you take them out of the refrigerator 15-30 minutes before serving. Unfrosted cupcakes may be stored at room temperature.