Preheat oven to 150°C (300°F) and line a baking sheet with baking paper.
Mix the butter, sugar, and vanilla together in a large bowl until smooth. Add the oil, stirring well.
Add half of the flour and all the baker's ammonia and mix the cookie dough until the flour is fully incorporated. Add the rest of the flour and again mix until the dough comes together - it will still be quite crumbly.
Use your hand to form the dough into about 20 small round balls, first pressing the dough together and then gently rolling it into round balls. Place the cookies on the baking sheet well spaced apart.
Bake for 15 - 20 minutes. Cool on the baking sheet for a few minutes before removing to a wire rack. Wait until the cookies are fully cool before eating.