In a large bowl or the bowl of a stand mixer, put the salted butter, tahini, golden caster sugar, soft brown sugar, vanilla bean paste and a small pinch of salt. Whip using electric beaters until fluffy, about 5-6 mins.

Crack in the egg and add the extra yolk, then beat to combine, about 2 mins. Sieve over the flour and bicarb, mixing gently until just combined, then mix through both chocolates. Chill the dough in the fridge for at least two hours.

Heat the oven to 190C/170C fan/gas 5. Use an ice cream or cookie scoop to measure out 12 balls of dough – roughly 65g each. Pour the sesame seeds into a shallow bowl and gently mix them. Roll each cookie dough ball in the sesame seeds. Put them onto two large, lined baking trays, spaced well apart as they will spread.

Bake for 13-15 mins until lightly golden, then remove from the oven and give the tray a good tap on the kitchen side. This helps create a crispy, chewy texture. Leave to cool completely on the tray before serving.