Blend the cottage cheese until completely smooth. In a large bowl, whisk the blended cottage cheese, tahini, melted butter, brown sugar, egg yolk and vanilla until thick and glossy.
Add the flour, protein powder, baking soda and salt, and stir until combined. Fold in the dark chocolate chunks.
Cover and chill the dough for 1 hour. Tahini doughs are soft at first, and chilling gives you thick cookies with gooey, nutty centers.
Preheat the oven to 350°F and line 2 baking sheets with parchment paper. Scoop the dough into 2 tablespoon balls and roll the edges in sesame seeds, leaving the tops bare.Place the balls 2 inches apart on the baking sheets.
Bake for 10 to 12 minutes, until the edges are golden and the centers look slightly soft. Sprinkle with flaky sea salt right away.
Cool on the baking sheets for 10 minutes, then move to a wire rack.