In a large mixing bowl, cream together the butter and brown sugar. Add the hazelnut liquor (if using) and vanilla, beat until light and fluffy, about 1 minute. Add the egg and mix until combined. Add the flour, baking soda, and salt, beat until combined and the dough forms a ball. If the dough feels too wet to roll out, add 2-4 tablespoons additional flour.Divide the dough in half. Roll out the dough on a floured piece of parchment to 1/4 inch thickness. Make sure you're using enough flour or your dough will stick. Cut out the cookies using your desired shape/shapes. I used a variety of snowflakes, a star, and a Christmas tree. Carefully transfer the cookies to a parchment-lined baking sheet. I recommend using a floured spatula to lift the cookies. Cover the baking sheet and place in the freezer. Freeze until firm, about 20-30 minutes. Roll out the leftover scraps, and repeat with the remaining dough. Preheat oven to 350° F. Bake the cookies on the middle rack of the oven for 8-10 minutes or until just lightly golden. Cool on the baking sheet 5 minutes, then transfer to a wire cooling rack to cool completely. Meanwhile, make the frosting. In a medium bowl, beat together the powdered sugar, hazelnut liquor (if using), vanilla, and 1 tablespoon water until smooth and drizzly. If needed, thin the glaze by adding water, 1 tablespoon at a time, until drizzly. To make the buttercream, in a bowl, beat the butter and cream cheese together until extra light and fluffy, about 3-4 minutes. Add the powdered sugar and vanilla. Beat together until whipped, about 2 more minutes.Decorate each cookie as desired with vanilla frosting, buttercream frosting, sprinkles, and or vanilla sugar.To Make Santas: Outline a star cookie with vanilla frosting, then fill the cookie in with frosting, leaving 1 triangle portion without frosting for Santa's face. Sprinkle/press red sprinkles and/or sanding sugar into the frosting to create his body and hat, shake off the excess. Attach two mini chocolate chips for his eyes. Fill in his beard with the buttercream frosting, then add a red sprinkle for his nose. See photos above.To Make Snowflakes: Pipe lines and dots of frosting onto each cookie to enhance the snowflake shape (see photos for visuals). If desired, sprinkle with vanilla sugar. Let the frosting set before serving, about an hour.To Make Christmas Trees: Fill in each tree with vanilla frosting, then sprinkle with vanilla sugar.