Preheat the oven to 180C/160C fan/gas 4.

Line a 12-cup muffin tin with paper cups.

Combine instant coffee, hot water, and milk in a bowl and let cool.

In a separate bowl, beat softened butter and light muscovado sugar until light and creamy (5-10 minutes).

Beat in eggs one at a time, ensuring each is fully incorporated before adding the next.

If the mixture curdles, add a spoonful of flour.

Sift in self-raising flour and gently fold until just combined.

Stir in the cooled coffee mixture and chopped walnuts.

Spoon the batter into the prepared paper cups, filling each about two-thirds full.

Bake for 15-20 minutes, or until a skewer inserted into the center comes out clean.

While the cupcakes bake, make the coffee syrup by dissolving instant coffee in boiling water in a small pan.

Add caster sugar and bring to a boil over medium heat.

Reduce the heat and simmer until the sugar dissolves completely and the syrup thickens slightly.

Remove from heat and stir in coffee liqueur (optional).

Once the cupcakes are baked, let them cool in the tin for 5 minutes.

Prick each cupcake all over with a cocktail stick.

Brush each cupcake with coffee syrup, allowing it to soak into the sponge.

Remove cupcakes from tin and let them cool completely on a wire rack.

Make the mascarpone frosting by beating mascarpone, vanilla extract, and icing sugar until creamy and well combined.

Add a dash of milk or cream if the frosting is too thick.

Spoon the frosting onto each cupcake and spread evenly with a palette knife.

Dust with cocoa powder, either all over or through a coffee-bean stencil.