Preheat oven to 180C degrees. Grease a loaf pan or 20cm round cake tin. Whisk together flour, baking powder and salt in a bowl.

In another bowl beat butter and sweetener with an electric beater or by hand until light and fluffy, about 5 minutes. Add eggs, one at a time, beating until combined after each addition. Mix in vanilla. Add flour mixture in 2 batches, alternating with the buttermilk and beginning and ending with the flour. Set aside 1/3 of the batter.

In a bowl, mix cocoa and 1/4 cup plus 2 tablespoons boiling water with a rubber spatula until smooth. Add the cocoa mixture to the reserved cake batter; stir until well combined.

Spoon batters into the prepared pan in 2 layers, alternating spoonfuls of vanilla and chocolate to simulate a checkerboard. To create marbling, run a table knife (or wooden skewer) through the batters in a swirling motion.

Bake, rotating the pan halfway through, until a cake tester comes out clean, 40 to 50 minutes. Transfer pan to a rack to cool 10 minutes. Turn out cake from pan and cool completely on the rack. Ice with butter icing and add sprinkles. Cake can be kept in an airtight container at room temperature up to 3 days.