Preheat oven to 375 degrees F. and line a baking tray with parchment paper.
Sift flour with baking powder and salt and set aside. Cream butter and sugar together until fluffy. Whisk the egg with milk and vanilla. Add flour mixture in three additions, alternating with milk mixture and blending after each addition. Wrap dough and freeze for 30 minutes.
On a lightly floured surface, roll out dough to just less than ¼-inch thick into rectangle, about 18 x 12-inches. Spread mincemeat evenly over pastry and trim any rough edges. From long end, roll up pastry to into a pinwheel. Freeze for 30 minutes.
Slice pinwheels into ½-inch and place an inch apart on a parchment lined baking sheet. Brush tops with egg wash. Bake for about 15 to 18 minutes, until a light golden brown.