Preheat the oven to 180°C/350°F/Gas Mark 4. Place 10 paper cupcake cases into a bun tin. Beat the butter and sugar together until light and fluffy, then gradually add the egg, beating well between each addition. Stir in the vanilla extract.
Sift the flour and baking powder together and fold one third into the mixture, then stir in one third of the milk. Continue this process until both are fully mixed in. Spoon the mixture into the cases (try using an ice cream scoop to keep the cupcakes the same size) and bake for 25 minutes.
When the cakes have cooled, inject with the jam using a syringe (available over the counter at chemists’ for administering children’s medicine). Alternatively, use a sharp knife to slice the top off each cake, fill the cavity with jam and then place the tops back on.
To make the topping, beat together the cream cheese or mascarpone, icing sugar and peanut butter until smooth. Pipe or swirl onto each cake and finish with a sprinkling of crushed peanuts.