Mix butter and sugar by hand or mixer, then gradually add the oil. Finally mix in flour and ammonium bicarbonate.
Roll into balls and place on a lined baking sheet. Flatten them a little with your fingers, then refrigerate for 15 minutes.
Bake for 20 minutes and allow to cool on the baking sheet.
NOTES :
Baker's ammonia can be found online or in select European or international markets. Using regular baking powder works, but they will not turn out as crumbly.