Make the Coffee Mixture: Dissolve 2 teaspoons of strong instant coffee granules in 1 tablespoon of boiling water, then set aside until completely cool.
Prepare the Oven and Tray: Preheat the oven to 180°C/160°C fan and line a 12-hole cupcake or muffin tray with large cupcake cases.
Cream the Butter and Sugar: Beat the 150 g unsalted butter or baking spread with the 150 g light brown sugar until the mixture is fluffy.
Add the Eggs and Flour: Add the 3 medium eggs and 150 g self-raising flour to the creamed mixture, then beat until smooth and evenly combined.
Add the Coffee: Pour in the completely cooled dissolved coffee and beat again until thoroughly incorporated.
Fold in the Walnuts: Add the 75 g chopped walnuts and gently fold them through the batter until evenly distributed.
Fill the Cases: Divide the cupcake batter evenly between the prepared cases.
Bake the Cupcakes: Bake at 180°C/160°C fan for 20–22 minutes, or until the cupcakes are cooked through.
Cool the Cupcakes: Transfer the baked cupcakes to a wire rack and leave them to cool completely before decorating.
Prepare the Buttercream Coffee: Dissolve the remaining 2 teaspoons of strong instant coffee granules in 1 tablespoon of boiling water and allow it to cool completely.
Loosen the Butter: Beat the 200 g unsalted butter on its own until loosened and smooth.
Add the Icing Sugar: Gradually add the 400 g icing sugar while mixing on a slow speed until combined.
Whip the Buttercream: Increase the mixer speed and beat for 3–4 minutes, until the buttercream is fluffy and smooth.
Add the Coffee: Pour the completely cooled dissolved coffee into the buttercream and continue beating for 5 minutes, until the frosting is whipped, light, and fluffy.
Decorate the Cupcakes: Pipe the coffee buttercream over the completely cooled cupcakes.
Finish with Walnuts and Sprinkles: Scatter the 50 g chopped walnuts and some sprinkles over the piped buttercream, then serve and enjoy.
