Bring the butter to room temperature. If it will take too long or the room is too cold, wrap it in plastic wrap and microwave on a low setting to soften. ※ Be careful to just soften, not melt, the butter.
Use a spatula to cream the butter until smooth. Change to a whisk and mix. Keep going until it gets kind of glossy and creamy, the consistency of mayonnaise.
Add the powdered sugar in about 3 batches. Mix well after each addition, and keep mixing until it's pale and fluffy.
Mix in the egg yolk and vanilla oil. Sift the flour into the mixture and cut it in.
When the mixture is no longer floury, use the spatula to bring the dough all together in one lump. The dough is now complete.
Wrap with plastic wrap and put into the refrigerator. If you can, chill for a whole day. If you don't have time, leave it in the refrigerator for at least 30 minutes to one hour. You can also make the dough in advance and store it in freezer.
Using a madeleine tin, I baked oyster shell shaped tart crusts.
A berry tart. I topped it with a variety of berries.
A Gateau Saint-Honoré. This is a classic French dessert that combines a tart crust with choux pastry (well, this is a vague approximation of that anyway)...
Here I made a baked cheesecake using this as the crust.
You can use it for so many other sweets as well, but this is the base recipe.