Preheat oven to 325 degrees, with rack on lower middle position. Line baking sheets with parchment paper or silicone mats, set aside
In a bowl of a stand mixer, add butter and brown sugar. Using a paddle attachment on medium-high speed, mix until light and fluffy, several minutes. Add only 3 ½ cups of the flour and mix on medium-low speed until combined
On a large board, use a rubber spatula to transfer dough to the board, knead dough by hand for 5 minutes, adding only as much of the remaining ½ cup flour as needed to make a soft dough, while continuing to knead by hand. The dough should end up soft and pliable, like play-dough, not sticky. Work in desired amount of sprinkles, if using
Divide dough into thirds, wrap tightly, and chill for at least 30 minutes or until firm enough to roll and cut shapes. Roll one-third of the dough out to about ½-inch thickness, lightly flour board if needed. Either cut 3x1-inch rectangular strips, triangles, or use cookie cutter for shapes. Place each cookie 2-inches apart on baking sheet. Poke cookies with fork to design if not using sprinkles
Bake 15-20 minutes, or until edges are golden brown. Repeat with remaining dough, working in batches so dough stays chilled and workable
Cool completely on wire rack before serving
