When the fruit is ready, whisk together the butter, buttermilk (or yogurt), egg and vanilla.
Pour the mixture into greased mini round cake pans – either springform, individual pans or a multi-well round pan – and bake for 12–15 minutes.
Alternatively, although it isn’t historically accurate, you can bake this in a small, greased cake pan like a 15cm loose-bottom round tin for approx. 35 minutes to make one satisfying small cake.