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Heat the oven to 180C/160C fan/gas 4. Butter a 20cm square tin and line with baking parchment. Melt the butter in large saucepan over a medium heat, then remove from the heat and stir in the sugar and half of the white chocolate. Leave to cool for 10 mins, then beat in the vanilla and beaten egg. Mix the flour, baking powder, milk powder (if using) and 1 tsp sea salt flakes or ½ tsp fine sea salt together in a separate bowl. Tip the dry ingredients into the wet, and stir well to combine, making sure no pockets of flour remain. Finally, scatter in the marzipan and remaining white chocolate, and stir these through the mixture.
Pour half the batter into a second bowl and stir in drops of pink food colouring until evenly coloured. Alternately spoon dollops of the pink and plain batters into the tin, then gently swirl together using a skewer or cutlery knife to create a swirling marbled effect.
Bake for 30-40 mins until the blondies are set at the edges with a slight wobble in the middle. Leave to cool completely in the tin, then cut into squares to serve. (If you like, chill overnight before cutting for neater squares.) Will keep in an airtight container for up to five days.