For the cookies: In a large bowl, beat together butter, both sugars, vanilla, and salt until creamy. Beat in egg yolk. Gradually beat in flour then stir in chocolate chips. Remove the dough from the bowl and place it between two sheets of plastic wrap. Roll the dough flat until it is 1/4-inch thick. Chill in the refrigerator for at least an hour or until firm.
Line two cookie sheets with either silicone mats or parchment paper. Remove the dough from the refrigerator and remove the top layer of plastic wrap. Cut into shapes and place shapes on the cookie sheets. As you run out of dough to cut, gather up the dough scraps and reroll. Repeat until all of the dough is used up. Chill the cookie sheets in the refrigerator while you preheat the oven to 350F.
When the oven is ready, bake the cookies 9-11 minutes or until golden brown. Let stand for 3 minutes then remove to wire racks to cool completely.
Meanwhile, prepare the vanilla buttercream. In a medium bowl, beat together sugar, butter and salt until creamy. Increase the speed to high and beat until creamy, 3-5 minutes. Add vanilla and beat for another minute.
When the cookies are ready, decorate with sprinkles and buttercream.
For the chocolate glaze: Place the chocolate chips in a plastic sandwich bag - do not seal. Microwave for 45 seconds then knead until smooth. Squeeze all of the chocolate to one corner of the bag then make a small cut to the corner. Pipe the chocolate onto the cookies as desired. Let the chocolate harden before serving.
Recipe by Chocolate Moosey at http://www.chocolatemoosey.com/2012/07/08/chocolate-chip-cut-out-cookies-for-sundaysupper/