In a large bowl, cream together the butter and brown sugar for 2 minutes. Add the egg and milk and beat to combine. Add the molasses and beat until fully combined. Set aside.
In a medium-sized bowl, stir together the flour, baking soda, cinnamon, ginger, and salt.
Beat the dry ingredients into the wet ingredients until no dry patches remain.
Cover with plastic wrap and place dough in the fridge for at least 1 hour (feel free to leave it overnight).
Preheat oven to 350F. Line two baking trays with parchment paper or silicone baking mats.
Add Christmas sprinkles to a shallow bowl.
Portion dough using a small cookie scoop, then roll in the Christmas sprinkles and place on the prepared baking trays, at least 2” apart.
Bake cookies for 10-12 minutes until spread and browned. Allow to cool for 3-4 minutes on baking sheets before transferring to a cooling rack to cool fully.
Notes
Store in an airtight container at room temperature. Best enjoyed within 2-3 days of baking.