In a medium bowl, using an electric mixer, beat the butter with the tahini, confectioners' sugar and salt at medium speed until smooth, about 3 minutes. Add the flour and beat until incorporated. Turn the dough out onto a work surface and knead just until it comes together.

Divide the dough in half and roll each piece into a 15-inch log, about 1 inch thick. Scatter the sesame seeds on a sheet of parchment paper and roll the logs until completely coated. Roll each log in parchment and refrigerate until chilled, at least 1 hour.

Preheat the oven to 325° and line 2 large baking sheets with parchment paper. Unroll one log and slice it 1/4 inch thick. Arrange the slices on the baking sheets. Bake the cookies in the center and lower-thirds of the oven for about 25 minutes, or until golden, shifting the pans from front to back and top to bottom halfway through baking. Slide the parchment onto a rack and let the cookies cool completely. Repeat with the second log of dough.

MAKE AHEAD The cookies can be kept in an airtight container at room temperature for up to 1 week.