To make cookies, in a large mixing bowl, combine butter, white sugar, and brown sugar until well-blended. Add in eggs, yolk, and vanilla. Mix until combined. In a large bowl, combine flour, baking powder, and cinnamon until well-blended. Add that dry mixture to the mixing bowl, and beat until well-blended. Chill the dough for at least 1 hour. In a small bowl, combine the sugar and cinnamon. Roll dough into 1-inch balls, and roll into the cinnamon-sugar mixture.

Place the balls about 2 inches apart on an ungreased baking sheet, and flatten slightly with a glass. Bake at 350 degrees for 8-10 minutes, or until very lightly browned at the edges, or if you tap the center of the cookie, it springs back. Remove cookies with a spatula to a wire rack, and cool completely.

To make frosting, in a large saucepan over low-medium heat, melt the white chocolate chips and butter. Whisk and mix until smooth. Whisk in vanilla. Carefully pour the mixture into a large mixing bowl. Beat in enough powdered sugar that the frosting is light, fluffy, and spreadable. Place a generous spoonful of frosting onto the bottom of one cookie, and form it into a sandwich with another cookie. Enjoy!