Step 1: Preheat your oven to 350°F (175°C) and line two large baking sheets with parchment paper.
Step 2: In a medium bowl, whisk together the flour, baking soda, and salt until well combined.
Step 3: In a large mixing bowl, cream the softened butter and granulated sugar together until the mixture is light and fluffy.
Step 4: Add the egg and vanilla extract to the butter mixture and beat until smooth. Stir in milk if the dough is too stiff.
Step 5: Gradually mix the dry ingredients into the wet ingredients until a soft dough forms.
Step 6: Roll the dough into 1-inch balls, then roll each ball in the colored sanding sugar until evenly coated.
Step 7: Place the balls 2 inches apart on the baking sheets and bake for 8-10 minutes until just set.
Step 8: Remove from the oven and immediately press a chocolate kiss into the center of each cookie. Transfer to a wire rack to cool.