Preheat the oven to 165 °C and prepare a cupcake tray by brushing the base and side of each mould with butter, then sprinkle a tsp of caster sugar in each.
Prepare the orange slices for the pan by removing the skin and slicing thin rounds of just the flesh (without the seeds) and place them in the cupcake tray over the sugar layer and set aside.
In a bowl, whisk together the butter and caster sugar until pale and fluffy. This can be done with a hand whisk too since this is a small batch recipe.
Add in an egg and whisk until combined.
To the bowl add in the flour, baking powder and mix. Sifting the flour and baking powder and flour is not needed in this recipe since again it is a small batch recipe.
Finally add in the roasted pistachio powder to the batter and fold to combine. Do not over mix the batter as you want a light and fluffy teacake.
Divide the batter into the 4 prepared cupcake moulds. Bake in the oven for 20 mins.
Additional info: After creaming the butter and sugar, feel free to add in the zest of one Orange or 1/2 tsp of Orange Blossom Water or Vanilla. All of these are optional of course but do add a special flavour to the cake so do try them if you like.