To start, you will need 2 greased 12-hole whoopie pie tins or a baking sheet lined with greaseproof paper. Preheat the oven to 180ºC/fan 160ºC/gas mark 4.
To make the pies, cream together the butter and brown sugar in a mixing bowl for 2–3 minutes using an electric hand-held mixer, until light and creamy. Add the egg and vanilla extract and mix again. Sieve the flour, cocoa and baking powder into the bowl and add the salt and yoghurt. Whisk again until everything is incorporated. Add the hot water and whisk into the mixture.
Put a large spoonful of mixture into each hole in the prepared tins, or pipe or spoon into 24 mounds on the prepared baking sheet. Leave to stand for 10 minutes then bake the pies in the preheated oven for 10–12 minutes. Remove the pies from the oven, let cool slightly then turn out onto a wire rack to cool completely.
Shortly before you are ready to serve, remove the ice cream from the freezer and allow to soften slightly. Cut 12 slices each about 2-cm thick and, using an 8cm biscuit cutter, stamp out a round from each slice. Sandwich an ice cream round between 2 cooled pie halves. Working quickly, put the sprinkles on a flat plate and roll each pie in them so that the ice cream is coated. Serve your whoopie pies immediately with napkins to catch any ice cream drips.