Preheat the oven to 350ºF. In a small bowl, whip together the room temperature butter and sugar with a fork.
Add the egg yolk, vanilla extract, and salt to the bowl with the butter and sugar. Whip with the fork again until the mixture is light and creamy.
Add the flour and baking powder to the bowl and stir together with a fork until everything is evenly combined. Divide the cookie dough into two equal-sized pieces, then roll each one into a ball.
In a separate small bowl, stir together the cinnamon and sugar for the topping. Roll each ball of dough in the cinnamon sugar until completely coated.
Place the cinnamon sugar coated cookie dough balls on a parchment-lined baking sheet. Bake the cookies for about 15 minutes or until they're mostly flattened, golden brown on the outer edges, and a little cracked on top.
Allow the cookies to cool slightly before enjoying.