Preheat the oven to 350 degrees F and grease a large ramekin well with butter.
Shortbread Crust - In a small bowl mix together the flour and powdered sugar then mix in the melted butter until combined. It should have a wet sand like consistency. Press the shortbread into the bottom of the greased ramekin and press down into an even layer.
Bake the crust for 12-14 minutes until just getting golden on the edges. Remove from oven and let cool for about 5 minutes.
Meanwhile, mix filling. Whisk the egg yolk and sugar in a mixing bowl. Add the lemon juice, lemon zest, and vanilla and whisk to combine. Whisk in the flour until smooth.
Once the crust has cooled for a few minutes, pour the filling over the crust.
Bake for 17-22 minutes. The filling should be just set and not jiggle when you shake the ramekin. Be careful not to overtake, remove when it has just set up.
Allow to cool. Either eat with a spoon or run a knife around the edge then tip ramekin upside down to remove. I like to dust with powdered sugar. Enjoy!