In a large mixing bowl using a wooden spoon, beat the butter and sugar together until pale. Beat in the egg, followed by the vanilla, the baking soda and the salt.
Add the flour, and stir gently until just combined. Chill the cookie dough for 30 minutes.
Meanwhile, line 2 large baking sheets with baking parchment, and place the sprinkles in a shallow bowl. Preheat the oven to 320F.
Roll the dough into walnut-sized balls, then roll each ball in the sprinkles before placing them at least 2 inches apart on the prepared baking sheets. Don’t worry about getting every last bit of the cookie coated with sprinkles.
Bake for 12 minutes until the cookies are slightly puffed and just ever so slightly golden around the edges. Allow to cool for 5 minutes on the baking trays before transferring to a wire rack to cool completely.
Notes
1. These cookies will keep in an air-tight container for up to 3 days.