In a mixer, combined butter, vanilla and both sugars until creamy. Mix in egg and salt. Add 1 cup of flour at a time, making sure it is well combined. The dough will be firm.
Cover the bowl and refrigerate for 30 minutes.
Place dough on a well-floured surface. Let warm about 5 minutes or until it can easily be molded without cracking.
Roll dough out to 1/4" thick.
Use a cookie cutter to cut dough. Place 1 tablespoon of mincemeat filling in the center of half of the cookies. Use the remaining rounds to cover the cookie and press edges together.
Lightly whip one egg white. Using a brush, coat the top and seam of each cookie with the egg white.
Cut small hole in the top of each cookie.
Bake at 350° for 20 minutes.