Preheat oven to 160°C. Grease base and sides of a 20cm x 30cm lamington pan and line with baking paper.
Place butter, cocoa, dark chocolate and half the milk chocolate in a heatproof bowl and set over a pan of gently simmering water (don’t let the bowl touch the water). Stir until melted and smooth, then set aside to cool.
Place sugar, vanilla, eggs and egg yolks in a bowl and whisk to combine. Fold in flour, milk powder and baking powder with ½ tsp salt, then fold in melted chocolate mixture and remaining 100g milk chocolate.
Spread batter into prepared pan and press pretzels into top. Bake for 45 minutes or until set but the centre is still fudgy. Cool brownies completely in pan, then freeze for 30 minutes or until firm before serving (I didn’t do the freezer part, I just let them cool!).