Preheat the oven to 180 C / Gas 4. Line a muffin tray with 12 paper cases.
Mix the cocoa powder and hot water in a small bowl and stir until smooth. In another large bowl beat butter and sugar until creamy. Add the eggs one by one and mix well after each addition. Fold in the flour and mix everything until just combined. Fill the mixture into the paper cases.
Bake in the preheated oven until firm to the touch, about 20 minutes. Transfer to a wire rack and let cool.
Melt the chocolate in a small saucepan or in the microwave. Stir in the nuts. Spread a a small about of the mixture on each cupcake. Fold the marshmallows and glace cherries into the remaining chocolate and spoon on top of the cupcakes. Leave until chocolate is set.