Heat the oven to 175 C / 350 F. Line a 33 x 46 cm / 13 x 18 inch sheet pan with parchment paper.

In a large bowl, whisk the tahini, butter, brown sugar, and 50 g granulated sugar together until smooth and creamy, about 1 to 2 minutes.

Add the egg and vanilla extract. Whisk again until the mixture looks glossy and slightly lighter in color.

In a separate bowl, stir together the flour, baking soda, baking powder, and salt.

Fold the dry ingredients into the wet mixture until just combined, then fold in the chopped pistachios. The dough will be soft, almost like a thick batter.

Cover the bowl and chill the dough for 30 minutes. This step matters, skip it and the cookies spread thin and run together on the pan.

Scoop the dough into 34 g balls, about 2 tablespoons each. Roll each ball in the extra granulated sugar until fully coated.

Place the balls on the sheet pan, spaced 5 cm / 2 inches apart. You'll need two batches to bake all 20 cookies without crowding the pan.

Flatten each ball slightly with the bottom of a glass. Press a few extra chopped pistachios on top and add a small pinch of flaky salt if using.

Bake for 11 to 13 minutes, until the edges are set and lightly golden but the centers still look soft. Cool on the pan for 5 minutes.

Move the cookies to a wire rack to cool completely before stacking or storing.