1 tsp ground cinnamon
In the bowl of a stand mixer, cream the butter and sugar until light and fluffy. Scrape down the sides of the bowl and beat in the egg followed by the egg yolk. Mix in the vanilla extract.
Bake for 20 minutes. Rotate the baking sheet and bake for an additional 20 minutes or until golden brown and firm to the touch.
Source: The Culinary Enthusiast