Preheat oven to 300°F.
In bowl of a stand mixer, beat together butter and brown sugar.
Add flour slowly until all ingredients are fully incorporated in bowl.
Place dough onto a floured surface and knead until smooth. Roll the dough out to 1/3" thickness.
Cut out triangle shapes using a cookie cutter.
Press a bottle cap (or other circle) into the dough. Use the back of a butter knife to add the line down the center of the cookie.
Place onto baking sheet lined with parchment paper (or wax paper). These cookies don't spread out in the oven, so they only need about 1/2" of space.
Bake for about 20 minutes, or until slightly golden and firm.
Remove from oven and allow to cool slightly before transferring to wire rack.