In the bowl of a stand mixer, add cold butter slices and brown sugar. Using paddle attachment on medium-high speed, cream until mixture is combined and has a pasty texture. Scrape down sides and paddle. Add only 3 1/2 cups of the flour and mix on medium-low speed just until combined; do not over-mix.

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, adding only as much of the remaining 1/2 cup flour as needed to make a soft dough, while continuing to knead. The dough should end up soft and pliable, similar to play-dough, not sticky. Gently work in desired amount of sprinkles, if using them.

Divide dough into 3 balls, and flatten them into discs. Wrap tightly with cling wrap and chill for at least 30 minutes or until firm enough to roll and cut shapes. Meanwhile, preheat oven to 325F, with rack on lower middle position. Line baking sheets with parchment paper or silicone mats; set aside

Take out one of dough discs and roll it out to about 1/2 inch thickness – lightly flour board if needed. Either cut into 3×1 rectangular strips, triangles, or use cookie cutters (I use 1-2" cookie cutters). Place cookies 2 inches apart on baking sheet. Poke cookies with fork on top for a design, if not using sprinkles. Place entire baking sheet in fridge for 15 minutes.

Bake chilled cookies for 15-20 minutes, or until edges are golden brown. For smaller cookies, bake closer to 15 minutes; bake longer for larger shapes. Repeat with the remaining dough, working in batches so dough stays chilled. Cool completely on wire rack before serving. Cookies keep very when sealed airtight, at room temp.