Heat oven to 180C/160C fan/ gas 4. Grease and line a 20cm square tin with baking parchment. Cut 100g of the chocolate into chunks and put to one side. Melt the butter with the remaining chocolate in a bowl over a pan of simmering water, then remove from the heat and beat in the sugar. Leave to cool to room temperature, then beat in the eggs one at a time. Fold the flour and matcha through the mixture and gently stir through the hazelnuts and chunks of white chocolate.
Bake for 25-30 mins – you want it to be a bit gooey. Allow to cool, cut into squares and serve.