Preheat the oven to 375 degrees and line a cookie sheet with parchment. In a small bowl, whisk together the flour, baking powder, 1/4 teaspoon cinnamon, the baking soda and salt. Using a mixer, cream 1 stick butter, the brown sugar and 1/4 cup granulated sugar until fluffy, about 2 minutes. Beat in the egg and vanilla. Mix in the flour mixture at low speed.
In a small bowl, mix together the remaining 2 tablespoons granulated sugar and 1/2 teaspoon cinnamon. Using a 1 1/2-inch scoop, form 12 mounds. Dip the rounded side of each mound into the sugar mixture to coat, then place sugar side up and 2 inches apart on the prepared cookie sheet. Bake the cookies until golden at the edges, 12 to 15 minutes. Let cool for about 2 minutes, then, using a spatula, transfer to a rack to cool completely.
Meanwhile, using a food processor, finely grind the hazelnuts. Add the confectioners sugar and the remaining 2 tablespoons butter; process until smooth. Spread 6 cookies with about 2 teaspoons hazelnut cream each and sandwich with the remaining 6 cookies.