Preheat oven to 325F, with rack on lower middle position. Line baking sheets with parchment paper or silicone mats; set aside.
In the bowl of a stand mixer, add butter and brown sugar. Using paddle attachment on medium-high speed, mix until fluffy and light, several minutes. Add only3 1/2 cups of the flour and mix on medium-low speed until combined.
On a large board, sprinkle 1/2 cup flour to keep dough from sticking to the board. Use rubber spatula to transfer dough to the board. Knead dough by hand for 5 minutes, adding only as much ofthe remaining 1/2 cup flour as needed to make a soft dough, while continuing to knead by hand. The dough should end up soft and pliable, similar to play-dough, not sticky. Work in desired amount of sprinkles, if using. (Important Note: You may not end up using all the flour, depending on how your dough feels; it’s important to stop adding flour as soon as your dough feels like soft play dough.)
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 1/2 inch thickness (lightly flour board if needed.) Either cut into 3×1 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. If your shapes are smaller, bake less time. Bake longer for bigger shapes. Repeat with the remaining dough, working in batches so dough stays chilled and workable.
Cool completely on wire rack before serving. Cookies keep very when sealed airtight, at room temp.