Preheat oven to 180°C conventional or 160°C fan-forced. Line a standard 12 hole muffin tin with cupcake cases.
Place the cake mix, butter, eggs and milk into a medium mixing bowl. Using an electric mixer, beat on low speed for 30 seconds to combine. Increase speed to medium and beat for 2 minutes, scrapping down the sides of the bowl occasionally.
Transfer half of the mixture to a separate bowl. Sift the cocoa powder over one half of the mixture and stir to blend well so that there is now one vanilla mixture and one chocolate mixture.
Line a standard 12-cup muffin tray with cupcake cases. Spoon one teaspoon (5ml) of vanilla mixture into the centre of each cupcake paper. Then, spoon one teaspoon (5ml) of chocolate mixture directly into the centre of the vanilla mixture. Continue spooning mixtures over one another, alternating between chocolate and vanilla, until it is all used up.
Bake for 20-25 minutes or until a skewer inserted into the centre of the cupcake comes out clean. Turn cupcakes out onto a wire rack to cool.
Make the frosting by placing the frosting mix, butter and milk into a small mixing bowl. Using an electric mixer, beat on low speed for 30 seconds to combine. Increase speed to medium and beat for 3 minutes, scrapping down the sides of the bowl occasionally. Spread over cooled cupcakes.