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Melt the butter and both sugars in a pan over a low heat, adding a pinch of salt if the butter is unsalted. Remove from the heat and leave to cool for 10 mins – this will stop the eggs from curdling when added. Heat the oven to 180C/160C fan/gas 4. Butter a 23cm square tin or 25 x 20cm baking tin and line with baking parchment.
Beat the eggs and vanilla into the cooled butter and sugar mixture until well-combined. Sieve in the flour and baking powder, then gently fold until you have a smooth batter. Fold in the white chocolate, then scrape into the prepared tin. Smooth the top and bake for 40-45 mins until the edges are coming away from the sides of the tin, the top has a papery crust and a skewer inserted into the middle comes out with a few crumbs, but no liquid batter. The blondies will firm up as they cool.
Leave to cool in the tin for 10 mins, then transfer to a wire rack to cool completely. The blondies will keep in an airtight container for three days.