Whisk the flour, salt, baking powder, baking soda, cinnamon, ginger, cloves, pepper and mustard together in a medium bowl.
Beat the butter and sugar together in a large bowl with an electric mixer on medium-high speed until just combined and lightly fluffy. Beat in the molasses and egg yolk. Gradually add the dry ingredients and mix together on low speed until dough is just combined and still crumbly, about 3 minutes. Give dough a few turns with a spatula to bring together.
Lay out wax paper on a clean work surface and put about 1/3 of the batter on top. Lightly press down and top with another sheet of wax paper. Using your hands or a rolling pin, gently pat into a rectangle.
Roll out with a rolling pin until dough is as thin as possible without breaking, no thicker than 1/16 inch thick. This is the key to these cookies: they really can't be too thin. Gently peel back the layer of waxed paper and then replace it loosely.
Transfer rolled batter to a flat baking sheet and freeze until firm and can easily be peeled away from the waxed paper, about 30 minutes. Repeat with the remaining dough.
Cut dough using a small (2- to 3-inch) fluted round cookie cutter and return to the freezer for 15 minutes to set.
Transfer frozen cookies to a baking sheet lined with parchment paper and bake until crisp and lightly, evenly colored (but not brown), about 10 minutes.
