In a large mixing bowl, cream the softened butter together with 3/4 cup of sugar until the mixture is light and fluffy. Incorporate the egg and vanilla extract, blending well. In a separate bowl, combine the flour, baking powder, and salt; gradually add this dry mixture to the creamed ingredients, stirring until fully combined. Gently fold in the minced mincemeat. Cover the bowl with plastic wrap and refrigerate the dough for two hours.

After chilling, scoop the dough and roll it into 1-inch balls. Dip each ball in the lightly beaten egg white, then coat it with the remaining sugar. Arrange the cookies on greased baking sheets, sugar side up, leaving two inches between each cookie. Bake in a preheated oven at 375°F for 10 to 12 minutes, or until the cookies are set. Once baked, transfer them to wire racks to cool.