Sift the flour, cream of tartar, baking soda / bicarbonate of soda into a bowl and keep aside.
In another bowl, combine the butter,oil and both granulated and powdered/ icing sugar. Beat in the vanilla extract and buttermilk and mix until smooth.
Add the dry ingredients to the wet ingredients and beat until combined, but do not overmix.
Using an ice cream or cookie scoop,drop the dough about 1 ½ inches / 4 cm apart onto a baking sheet lined with parchment / baking paper.
Bake for 8 to 12 minutes at 356°F / 180°C until the bottom and edges of the cookie is lightly browned. Remove from the oven and leave on the baking sheet for 5 minutes before removing and placing onto a wire cooling rack and allow to cool completely before frosting.
FOR THE FROSTING / ICING
Whisk the butter, vanilla and milk together.
Beat in the powdered / icing sugar and mix until combined and smooth.
The frosting / icing should be of a thick, spreadable consistency. Spread the frosting / icing over the cookie and decorate as preferred.
1.Cream of tartar is one of the ingredients in this recipe. Although just one teaspoon of cream of tartar is used, it is this ingredient which helps create that puffy, tender texture of this cookie. So please do not leave it out. Cream of tartar is easily found in the baking aisle of your grocery store. It is usually found with similar ingredients like baking powder, baking soda / bicarbonate of soda and tartaric acid.2. If you do not have buttermilk, use ONE of the following as a substitute: Add 1 tablespoon lemon juice or vinegar to 1 cup of milk. Allow to stand for 5 minutes before using in the recipe OR Use 1 cup plain unflavoured yoghurt