Preheat the oven to 170°C/gas 3. Place 18 cupcake liners in cupcake tins (or 48 mini cupcake liners in mini cupcake tins) and place three raspberries in the base of each (only use one raspberry if using mini cupcake liners).
In a large bowl, cream together the butter and sugar. Add the eggs, slowly, mixing until light and fluffy. Sift the flour and baking powder into the batter and fold in carefully. Stir in the milk, rose water and finally, the pistachios.
Fill a large piping bag with the batter, snip the end and pipe the batter into the cupcake cases until they are two-thirds full. Bake for 15 minutes until risen and golden brown. Remove from the oven and transfer to a wire rack to cool.
To make the buttercream, whip the butter for 1–2 minutes until soft. Add the icing sugar in two batches and whisk until fully combined. Add the milk and raspberry powder and whisk. Top each cupcake with a swirl of buttercream and decorate with chopped roasted pistachios.
Store for two days in an airtight container at room temperature (not suitable for freezing).