Line an 8" square cake tin with baking or greaseproof paper, and pre-heat the oven to 160 degres C (fan assisted) (180 degrees C, 350 F or gas mark 4)
In a small saucepan gently melt the butter over a low heat.
Add the white chocolate to the melted butter and slowly stir until it's all melted. This mixture may separate, leaving the melted butter on top, but that is ok. Once melted, take it off the heat and leave to cool slightly.
Break your eggs into a bowl and lightly beat.
Add in both the caster and light brown sugar and beat until it has gone slightly paler in colour and has thickened. This should take about 2 minutes by hand. Less if you use an electric mixer.
Add in your Vanilla Extract and Lemon Zest and stir to mix it through.
Add your flour and fold it through your mixture, make sure to mix in any lumps of flour to get to a smooth mixture.
Pour your mixture into your cake tin, then take the lemon curd and dot or drizzle it across the top of your blondies. You can use a skewer or end of the spoon to drag and swirl the curd across the top.
Put the blondies in your oven and bake for 30 minutes. When baked, the top should be lightly golden. Leave them to cool before cutting for cleaner slices.