In a bowl, mix together butter, essence and sugar till smooth. Don't beat till light and fluffy.
Sift together whole wheat flour/all purpose flour, cocoa powder and baking powder. Add a pinch of salt if using unsalted butter.
Add to the butter and sugar mix, and mix together till it comes together. Don't over mix it, the crust will become hard.
Arrange the dough into loose-bottomed tart shells. You can use one big tart pan also. This dough will give you 4 tart shells. If you double the recipe you will get 8 tart shells.
Keep all the shells in refrigerator for at least half an hour.
When ready to bake, preheat the oven to 325F or 170C. Bake the shells for 10-12 minutes.
Once baked, take out of the oven, after 2-3 minutes when shells start to cool, take out very carefully and cool completely on a wire rack. Don't try to take the shells out when warm, as these are very soft and can break while taking out.
Keep in an air-tight container in the refrigerator, till ready to use. These tart shells can be stored for 3-4 days.