Melt the butter, brown sugar and caster sugar in the same pan over low heat. Add in a pinch of salt if the butter is unsalted. Remove from heat and allow to cool for 10 minutes.
Heat the oven to 180 degrees Celsius, and butter or line a 23-centimetre baking tin with parchment paper.
Beat the eggs and vanilla into the cooled butter and sugar mixture until well combined. Sprinkle in the flour and baking powder, using a sieve if you have one. Gently fold until you have a smooth batter, then add in the white chocolate chips.
Pour the batter into the tin, and smooth the top. Bake for 40 to 45 minutes until the edges come away from the sides and the top has a papery crust. If they still seem gooey in the middle, they will firm as they cool. Let cool for 10 minutes, then transfer to a wire rack.
