Cream the base: In a large bowl, beat butter, cream cheese, and granulated sugar until light and fluffy, about 2–3 minutes. Mix in the egg and vanilla until smooth.

Combine dry ingredients: In a separate bowl, whisk flour, baking powder, and salt. Add to the wet mixture and mix just until a soft dough forms. Do not overmix.

Chill the dough: Divide dough in half, flatten into disks, wrap, and chill for 45–60 minutes. Cold dough rolls and cuts cleanly.

Roll and cut: Preheat oven to 350°F (175°C). On a lightly floured surface, roll dough to about 1/4-inch thickness. Use a small tree-shaped cookie cutter (about 2.5–3 inches) to cut pairs of cookies. Reroll scraps as needed.

Bake: Place cookies on a parchment-lined sheet, 1 inch apart. Bake 8–10 minutes, until set and just barely golden on the bottom. Let them cool completely on a rack.

Make the filling: Beat butter and shortening until creamy. Add confectioners’ sugar, vanilla, and a pinch of salt. Mix, adding milk 1 tsp at a time, until spreadable and fluffy. You want a thick but soft filling.

Assemble sandwiches: Spread 1–2 teaspoons of filling on the flat side of half the cookies. Top with remaining cookies to make sandwiches. Chill for 15 minutes to help them set.

Melt the coating: In a microwave-safe bowl, melt white chocolate or candy melts in 20–30 second bursts, stirring between each, until smooth. Add a teaspoon or two of vegetable oil if it seems too thick.

Dip or spoon-coat: Working with a few at a time, dip each sandwich cookie into the white coating or spoon it over the top. Let excess drip off, then place on parchment.

Decorate: While the coating is still tacky, drizzle with red icing in zigzags and sprinkle with green sanding sugar or festive sprinkles. Let set at room temperature or chill for faster setting.