Preheat your oven to 190C (170C fan) and line two baking trays with parchment paper.
Place the flours, baking powder, bicarbonate of soda and salt into a large bowl and mix together to combine, then set aside.
Put the butter, tahini and sugar into a large bowl and using an electric whisk, beat together until light and fluffy. This should take about 5-7mins.
Add the egg, egg yolk and vanilla and beat again to combine.
Scrape down the sides of your bowl using a spatula, then pour in your dry ingredients. Mix with your whisk until a dough starts to form.
Add your chopped chocolate and use your hands to evenly distribute.
If resting your dough to deepen the flavours, cover the bowl with clingfilm and refrigerate for 4 hours or overnight.
Weigh the dough out using a portioner or scales into 50g balls, then roll in a bowl of sesame seeds to cover.
Place onto parchment-lined baking trays, a few inches apart as they will spread and sprinkle with the flaky sea salt.
Bake for about 25 minutes until golden around the edges, puffed up and still a little soft in the centre. Allow the cookies to cool on a baking tray for 10minutes before transferring to a wire rack to cool completely.
Notes
Store in an airtight container for up to 3-4 days.