Beat the butter and sugar together until light and fluffy. Add the egg and vanilla, then mix until smooth. In a separate bowl, whisk the flour, baking powder, baking soda, and salt. Add the dry mix to the butter mixture and blend until a soft dough forms.

Roll the dough into eight large balls. Gently press the tops with your palm to form thick discs. Place on a lined baking sheet and bake at 350°F (175°C) for 10–12 minutes. The cookies should look set on the edges but pale in the centre. Let them cool completely on the tray.

Beat the butter until creamy. Add powdered sugar gradually, then mix in vanilla and milk until smooth and spreadable. Add food colouring if you want the classic Crumbl shade.

Spread a generous swirl of frosting over each cooled cookie. Let them sit for 10–15 minutes so the frosting firms up slightly.