Make the dough: In a medium bowl, whisk the flour, baking powder, and salt. In a large bowl, beat the butter and sugar until light and fluffy, about 2–3 minutes. Add the egg, vanilla, and almond extract. Mix in the dry ingredients just until combined. The dough should be soft but not sticky.

Chill: Divide the dough in half, flatten into two disks, and wrap each in plastic. Chill for at least 1 hour. Chilled dough means cleaner edges and less spread.

Roll and cut: Preheat the oven to 350°F (175°C). On a lightly floured surface, roll the dough to about 1/4 inch (6 mm) thickness. Cut into shapes with cookie cutters. Re-roll scraps as needed.

Bake: Place shapes on a parchment-lined baking sheet. Bake 9–12 minutes, until the edges are just set and pale golden on the bottom. Do not overbake. Let cookies rest on the sheet for 5 minutes, then transfer to a rack to cool completely.

Make the royal icing: In a stand mixer bowl, whisk powdered sugar and meringue powder. Add 6 tablespoons water and vanilla. Beat on low to combine, then on medium for 2–3 minutes, until thick, glossy, and slightly stiff. Add water a teaspoon at a time until the icing reaches piping consistency: it should form soft peaks that hold for a few seconds.

Divide and color: Split icing into bowls. Add gel colors a little at a time. For outlining, use thicker icing. For flooding (filling in), thin with water until it flows smoothly back into itself in about 10–12 seconds. Keep bowls covered with a damp towel to prevent crusting.

Outline and flood: Transfer piping-consistency icing to piping bags fitted with small round tips (or use snipped-tip plastic bags). Outline the cookies and let set for a few minutes. Thin icing for flooding goes into separate bags or bottles. Flood inside the outline, then nudge icing to the edges with a toothpick or scribe tool.

Add details: For wet-on-wet designs (like polka dots or marbling), add them immediately while the base is still wet. For sharp details (like lines, snowflakes, or lettering), let the base dry 1–2 hours, then pipe details with thicker icing.

Dry completely: Let cookies dry uncovered at room temperature until the icing is fully set, 6–12 hours or overnight. Avoid humidity if possible.

Finish and store: Once dry, add any edible glitter, luster dust, or small sugar decorations. Package for gifting or store as noted below.