Chocolate Whoopie Pies

Preheat the oven. Heat the oven to 180°C (160°C fan) with shelves set in the upper and lower thirds, giving it at least 20 minutes to reach temperature. Having two shelves ready means both trays can go in together.

Cream butter and sugar. Beat the butter and brown sugar together for 3 minutes until fluffy, then beat in the egg and vanilla. The brown sugar's molasses content helps keep these cakes moist as well as sweet.

Mix in the dry ingredients. Whisk together the flour, cocoa, bicarbonate of soda and salt, then mix into the batter in three additions, alternating with the buttermilk in two, on low speed until smooth. The batter should be thick, more like a soft icing than a pourable cake mix.

Scoop the mounds. Scoop 24 level mounds of batter, about 2 tbsp each, onto the lined trays, spacing them 5cm apart. The gap gives each mound room to spread and dome without merging into its neighbour.

Bake until domed. Bake for 10-12 minutes, swapping the trays between shelves halfway through, until the cakes are domed and the tops spring back when pressed. Swapping the trays evens out any difference in oven heat between the shelves.

Cool the cakes. Leave the cakes on the trays for 5 minutes, then transfer to a wire rack to cool completely. Filling them while warm would melt the buttercream straight into the crumb.

Make the filling. Beat the butter for 3 minutes until pale, then add the icing sugar on low speed. Mix in the cream, vanilla and salt, then beat for 3-4 minutes until very light and fluffy. The long beating time whips extra air in, so the filling stays soft rather than dense.

Sandwich the pies. Pair up the cooled cakes by similar size, then pipe or spread a thick layer of filling onto the flat side of half of them. Top with the remaining cakes to sandwich, pressing gently so the filling meets the edges.