Preheat your oven to 350℉. Grease your muffin tin and dust it with flour lightly to keep the shells from sticking when they bake.
Melt your butter. Cream together the butter, sugar, and vanilla extract.
Add the flour and mix until combined. You're looking for it to be a crumbly, greasy dough that holds its shape when pressed together. If it's too dry, add in a tsp of ice water at a time until you get it. If it's too moist, add a bit of flour at a time.
Spoon about a tablespoon of dough into each muffin cup. Use something like a shot glass or your fingers to flatten and press it around the edges to create a "cookie cup" shape.
Bake the crusts for 12 minutes, or until they start to brown. Remove from the heat and, if they've puffed up, gently flatten them with a spoon or other relatively flat instrument.
Set the shells aside and let them fully cool. Once cooled, you should be able to gently rotate them to loosen them and lift them out to be filled and decorated however you like.
If using the lemon curd from my previous recipe (linked in the post), you can optionally re-bake the curd-filled shells for a thicker texture and mellower flavor. Bake at 350 for about ten minutes, then let cool again.