Preheat oven to 190°c and line two large baking trays with baking paper. In small bowl, whisk flour, baking soda, and salt until combined.
In a large bowl using a flat beater attachment, beat butter and peanut butter until smooth. Add sugars and beat until light in color and fluffy. Beat in egg, then add milk and vanilla and mix until combined. Gradually add flour mixture, stirring on low, until just combined.
Roll a tablespoonful of cookie dough into a ball, then roll in sugar. Place on baking sheet and repeat with remaining dough, spacing cookies at least 5cm apart.
Bake until cookies have risen and look a little crisp on the outside, about 8 to 10 minutes. (They won’t brown!)
Immediately top with the chocolate kisses before serving.