• Line a large tray or two with baking paper.
• In a large bowl, beat the butter and sugar together until smooth. Then mix in the vanilla extract and the egg.
• Add in the flour, baking powder and salt. Mix well and when it starts to stick, use your hands to press everything together into a ball of dough.
• Pop the ball of dough in the fridge for 30 minutes to chill.
• While the dough is chilling, preheat your oven to 180°C/350℉ (or 160°C/320℉ for fan or convection ovens).
• Gently knead the cookie dough in your hands to get rid of any cracks then roll the dough out on a lightly floured surface until it's around ¼-inch thick.
• Use a 3-inch round cookie cutter to cut out your cookies, re-rerolling leftover pieces of dough until it's all used up.
• Bake for 10-11 minutes, until they are firm around the edges and slightly soft in the middle. They will firm up more as they cool.
• Leave them on the tray to cool completely.

Topping:

• Mix the butter, icing sugar, vanilla extract and milk together until smooth then stir in the green food colouring.
• Use a teaspoon to gently spread some icing onto each cookie then top with Lucky Charms marshmallows.
• Let the icing harden then enjoy!