Step 1In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl using a hand mixer, beat the butter and sugar until light and fluffy, 3 to 5 minutes.
Step 2Add the Tahini and beat just until combined. Then add the molasses and beat until incorporated.
Step 3In a separate bowl, whisk together the flour, baking powder and salt. Gradually beat the dry ingredients into the butter mix until fully incorporated to form the dough.
Step 4Wrap the dough in plastic wrap and refrigerate until firm, 20 to 30 minutes. (The dough can be made ahead of time and refrigerated up to 3 days before using.)
Step 5Meanwhile, heat the oven to 350 degrees. Line 2 to 3 baking sheets with parchment paper, and place the sesame seeds in a bowl.
Step 6Remove and unwrap the chilled dough. Portion the dough, a tablespoon at a time, and roll each portion into a ball. Roll each ball in sesame seeds, then lightly press down. Place the cookies on the baking sheet, spacing each about 2 inches apart (the cookies will spread as they bake).
Step 7Bake the cookies until golden, 9 to 11 minutes. The cookies will be very fragile when they first come out of the oven. Cool completely on the sheet before moving.