Make the sugar cookies first. Whisk flour, baking powder, and salt. In another bowl, beat butter and sugar until light and fluffy, about 2 minutes.
Mix in egg yolk and vanilla. Add dry ingredients and beat just to combine. Form a disk, wrap, and chill 30–45 minutes.
Cut and bake the stars. Heat oven to 350°F (175°C).
Roll dough to about 1/4 inch on a lightly floured surface. Cut small stars and place on a parchment-lined sheet. Bake 8–10 minutes until edges are set and pale golden.
Cool completely.
Prepare the cupcake batter. Line a 12-cup muffin tin with paper liners. Whisk flour, baking powder, and salt. In a separate bowl, beat butter and sugar until fluffy, 2–3 minutes.
Add eggs one at a time, then vanilla and almond extract.
Finish the batter. Mix milk and sour cream. Add dry ingredients to the butter mixture in two additions, alternating with the milk mixture. Beat on low until just smooth.
Don’t overmix.
Bake the cupcakes. Divide batter evenly among liners, about two-thirds full. Bake at 350°F (175°C) for 18–20 minutes, or until a toothpick comes out clean. Cool in pan 5 minutes, then transfer to a rack to cool fully.
Make the buttercream. Beat butter on medium-high until creamy and pale, 2 minutes.
Add 3 cups powdered sugar, vanilla, and 1 tbsp cream. Beat until smooth and fluffy, adding more sugar or cream to reach a thick, pipeable consistency. Tint with green gel, starting with a small amount and adjusting to your preferred shade.
Pipe the “trees.” Fit a piping bag with a large star tip.
Hold the bag vertically over a cooled cupcake. Pipe a tight swirl starting at the center, stacking upward to create a cone. Maintain gentle pressure and lift slightly as you go to build height.
Decorate. Add sprinkles as ornaments while the frosting is soft.
Top each tree with a sugar cookie star. A pinch of edible glitter gives a snowy shimmer.
Let them set. Chill the decorated cupcakes for 10–15 minutes to help the buttercream hold its shape, especially if your kitchen is warm.