Preheat your oven to 350°F (180°C) and line a muffin tin with cupcake liners. This recipe makes approximately 6 cupcakes.

In a mixing bowl, whisk together the flour, sugar, cocoa powder, baking powder, baking soda, and salt until well combined.

In another bowl, whisk the egg, then add milk, vegetable oil, and vanilla extract. Mix well.

Mix the espresso powder into the dry ingredients and add the hot water to the wet ingredients. Stir until smooth.

Gradually pour the wet mixture into the dry mixture and stir until just combined. Be careful not to overmix; a few lumps are okay.

Divide the cupcake batter evenly among the cupcake liners, filling each about 2/3 full.

Place the muffin tin in the preheated oven and bake for approximately 18-20 minutes or until a toothpick inserted into the center of a cupcake comes out clean.

Remove the cupcakes from the oven and let them cool in the muffin tin for a few minutes, then transfer them to a wire rack to cool completely.

While the cupcakes are cooling, prepare the frosting. Chop the dark chocolate into small pieces and place it in a heatproof bowl.

Microwave the cream until hot but not boiling, then pour it over the chopped chocolate. Add the butter. Let it sit for a minute to melt the chocolate.

Allow the frosting to set until it reaches the texture of peanut butter.

Once the frosting has reached the desired texture, transfer it to a piping bag fitted with a star tip.

Pipe swirls of frosting onto each cupcake.

Sprinkle rainbow nonpareils on top of the frosting for a colourful birthday touch!