Heat the oven to 180C/160C fan/gas 4 and line a 15 x 25cm cake tin. Put the pistachios in a food processor with half the sugar and blitz until the nuts are finely chopped. Tip into a bowl, add the remaining sugar and the butter and beat with an electric whisk until creamy. Add the eggs one at a time, then beat in the flour and yogurt until smooth, adding some milk if it’s too thick.
Spoon the batter into the tin, level and bake for 50-55 mins, or until a skewer inserted into the middle comes out clean. Leave to cool in the tin for 15 mins, then turn the cake out onto a board. Remove and discard the baking parchment, then leave to cool completely. Cut into neat squares.
To make the icing, put the raspberries in a pan with 2 tbsp water and cook briefly until soft and juicy. Push the mixture through a sieve and discard any pips. Whisk in the sugar until you have a smooth pink icing.
Pour the icing over the cooled cake, then decorate with the sugar flowers, pearl sugar or sprinkles, if you like. Leave to set before serving.