Blend the fresh mint leaves with the sugar until fragrant and finely ground.
Beat the softened butter until creamy, then mix in the mint sugar.
Add the egg and mix until smooth.
In a separate bowl, combine the flour, baking powder and salt.
Gradually add the dry ingredients until a soft dough forms.
Chill the dough for 30–40 minutes.
Roll into walnut-sized balls and gently flatten each one.
Bake at 350°F (180°C) in PREHEATED OVEN for 10–13 minutes, until the edges are set but the centers are still soft.
Let the cookies cool on the baking tray for 10 minutes, then transfer to a wire rack.
