In a large mixing bowl or the bowl of a stand mixer, pour in the tahini and mix well. Add the butter and sugar then mix using an electric whisk or stand mixer fitted with a paddle attachment. Mix on medium for a few minutes, or until light and fluffy. This can be done by hand, but will take much longer.
Add the egg and vanilla and mix for another few minutes until well incorporated.
Meanwhile, add the sifted flour, baking powder, bicarbonate of soda and 1 tsp salt to a bowl and mix to combine. Pour into the bowl with the wet ingredients and mix at a low speed, or fold in with a spatula until just combined.
Finally fold in the chocolate using a spatula.
Place the cookie dough in the fridge for up to 12 hours, but at least 2 hours (or freeze for 30-60 minutes).
Once sufficiently chilled, preheat the oven to 220C/200C Fan/Gas 7) and line a baking sheet with greaseproof paper.
Use a tablespoon to scoop the dough and form into spheres roughly the size of golf balls. Place on the baking sheet at least 1.5 inches apart allowing them space to expand. If you don’t have enough space, bake in batches or on 2 sheets.
Place in the oven and bake for 8-10 minutes, turning the tray around midway through cooking. The cookies will be cooked when fairly pale in the centre but browned around the edges.
Remove the cookies from the oven, sprinkle with flaky salt and the sesame seeds, then let cool on a rack for at least 30 minutes.
