In a large mixing bowl or bowl of a stand mixer, add butter and sugar and mix on medium-high speed until light and fluffy. Add flour and a pinch of salt, and mix on low until the mixture resembles breadcrumbs.

Use your hands to work in the rainbow sprinkles, and continue kneading gently until a loose dough has formed.

Shape dough into a log, approximately 8 inches (20 centimeters) long and about 1 1/2 inches (4 centimeters) in diameter.

Roll the log* in more rainbow sprinkles, pressing down gently to make sure they adhere to the dough.

Wrap log in clingfilm/plastic wrap and chill in the refrigerator until firm, at least 1 hour.

While dough chills, preheat oven to 190° C / 375° F. Line a baking sheet with a silicone baking mat, and set aside until ready to use.

Unwrap log and slice into 1/2 inch (1 1/4 centimeter) thick slices. Ttransfer to prepared baking sheet and sprinkle with additional sprinkles, pressing gently into the tops of the slices.

Transfer tray to the freezer and freeze for 10 minutes (or chill in the fridge for 30 minutes if your tray doesn’t fit in the freezer!)

Bake in preheated oven until shortbread is just lightly golden around the edges, about 13-15 minutes. Remove from oven and transfer to a wire rack to cool completely.

Store shortbread biscuits in an airtight container at room temperature for up to five days.