Preheat the oven to 180C.
Make the flax egg by combining flax and water and allow it to sit for 5 mins.
Beat butter and sugars together in a bowl with electric beaters until light and fluffy. Beat in flax egg and vanilla, then fold in the flour, baking powder, rolled oats and chocolate chips. Form 1 tbsp mixture into a ball and flatten slightly with the palm of your hand. Place on a lined baking tray and bake for 15min. Allow to cool completely.
Once cooled, match the cookies in pairs of similar sizes. Top one cookie with a scoop of Halo Top Chocolate Chip Cookies dough and press another cookie on top to sandwich together. Serve immediately or store in the freezer until needed!