Preheat the oven to 200C/180C Fan/Gas 6. Line an 8 inch square baking tin.
Combine the melted butter, granulated sugar and light brown sugar in a large mixing bowl or the bowl of a stand mixer. Combine on high speed for 3-5 minutes.
Add the eggs and vanilla and combine again.
Fold in the flour, baking powder, white chocolate chunks and cinnamon, using a wooden spoon/silicone spatula.
Pour the batter into the prepared tin. Stir together the 'drizzle' ingredients and drizzle over the batter. Bake for 30 minutes, until golden on top. Let cool on a wire rack.
Once cool, make the glaze by combining the icing sugar and milk in a small bowl and drizzle over the blondies. Let set about 10 mins before slicing and serving.