Heat the oven to 325 degrees F. Butter a 9-inch cake pan and set aside.
Beat butter, brown sugar and vanilla extract together until fluffy, about 3 minutes.
Add flour in 4 batches and combine well after each addition. This can be mixed
with your hands. Scrape the dough into the prepared pan and pat into an even layer.
Prick the surface with the tines of a fork. Score the top (without cutting all the
way through the dough), so that it will be easy to break apart into serving pieces.
Bake in the upper third of the oven for about 30 minutes, or until the top is
puffy and lightly browned.