Put the raisins in a small pan and cover with the brandy or rum. Heat briefly over a low heat until just steaming, then remove from the heat and leave to cool completely – the raisins will plump up as they cool.
Heat the oven to 180C/160C fan/gas 4. Butter the base and sides of a 20cm square cake tin, then line with baking parchment.
Melt the butter and 200g chocolate in a heatproof bowl set over a small pan of simmering water, or in 30-second bursts in the microwave, stirring until melted and smooth. Remove from the heat and leave to cool.
Beat the eggs and both sugars in a medium bowl with an electric whisk until pale and fluffy, about 3 mins. Stir the raisins and the liquid from the pan into the chocolate mixture (don’t worry if it looks like it’s starting to split), then fold into the egg mixture bowl using a large metal spoon. Do this gently, as you want to keep as much air in as possible. Sift over the flour and cocoa powder, and gently fold in the remaining chocolate.
Pour the mixture into the prepared tin and smooth the surface using the back of a spoon. Bake on the middle shelf of the oven for 35-40 mins until set with a very gentle wobble in the middle, and slightly risen around the edge. Leave to cool in the tin for 15 mins.
To make the brandy butter, mash the butter and sugar together in a small bowl until soft and smooth, then stir in the brandy or rum. Prick the warm brownie all over with a cocktail stick, then spread the brandy butter over the top so the butter melts into the brownie. Leave to cool in the tin for at least 20 mins more – the butter will form a crispy crust as it cools, but will be more like a glaze while the brownie is warm. Serve warm, or leave to cool to room temperature. Cut into small squares and serve piled up on a plate. Will keep, covered in the fridge, for up to three days.