Preheat oven to 180 C / Gas 4. Grease a 12-cup muffin tin or line with paper cases.
Sift flour, cocoa powder, bicarb of soda and salt together in a bowl.
Combine sugar and butter in another bowl; beat with an electric mixer until light and fluffy. Beat in egg and vanilla extract. Mix in flour mixture alternately with milk. Beat until just blended.
Bake in the preheated oven until tops spring back when lightly pressed and a toothpick inserted into the centre comes out clean, 16 to 20 minutes. Remove from oven and cool on a wire rack for 15 minutes. Remove cupcakes from tin and cool completely, about 2 hours.
Cover each cupcake with a thin layer of vanilla icing. Place 1 marshmallow half, cut-side up, in the centre of each cupcake.
Dust a work surface with icing sugar. Pinch off walnut-size pieces of white fondant; roll into thin circles that are slightly bigger than the cupcakes.
Drape over the marshmallows so they look like ghosts.