Heat the oven to 160C (140C fan)/325F/gas 3. Grease and line a 23cm square brownie tin with parchment so that it comes up the sides.
Melt the butter in a saucepan and leave it to cool slightly.
Beat the eggs, sugar and vanilla until the mixture is a little lighter in colour and starting to froth, then beat it into the butter until you have a smooth, glossy batter.
In a separate bowl, whisk together the flour, baking powder and salt, then fold into the egg mix, along with half the chocolate.
Spoon the batter into the tin, smooth the top with a spatula, spoon over the tahini, then run a table knife up and down the surface a few times to swirl the tahini into the batter a little.
Gently press the remaining chocolate pieces into the batter, and bake for 25-30 minutes – there should still be a slight wobble in the top of the mixture. Leave the blondies to cool in the tin for 15 minutes, then transfer to a wire rack to cool completely.
Drizzle with extra tahini, sprinkle with sea salt, slice and serve.