Beat the butter and tahini until soft and combined in a stand mixer or using an electric whisk. Tip in the sugars and beat again until light and fluffy, about 5 mins. Crack in the eggs, one by one, beating well after each addition. Mix in the vanilla, flour and bicarbonate of soda until no pockets of flour remain. Stir in the chocolate, then cover the bowl and chill overnight (for at least 18 hrs). Will keep chilled for up to three days. Or, roll into a log or balls and freeze for up to three months.

Heat the oven to 180C/160C fan/gas 4 and line two large baking trays with baking parchment. Roll the dough into cherry-tomato-sized balls, about 50g each. Arrange on the baking trays, spaced well apart, and press down on each one lightly with the palm of your hand to flatten them slightly. Sprinkle over the sesame seeds and a pinch of sea salt flakes. Bake for 12-14 mins until the edges are golden and set, and the middle is still gooey. Leave to cool completely on the trays before serving. Will keep in an airtight container for three days.