Preheat and Prep Pans: Heat oven to 350°F. Line baking sheets with silicone mats or parchment paper.
Cream Butter and Sugar: In a large bowl, beat the softened butter and sugar with an electric mixer on high speed until fluffy and pale, about 1 minute.
Mix Egg, Vanilla, and Leavening: In a small bowl, whisk together the egg, vanilla, baking powder, and salt. Add this mixture to the butter-sugar mixture and beat until smooth.
Add Flour Gradually: Add the flour 1 cup at a time on low speed, scraping the bowl as needed. The dough will look dry at first.
Bring Dough Together with Cream: Beat in the heavy cream for 10–15 seconds until the dough comes together, soft but not sticky.
Roll Between Wax Paper: Divide the dough in half. Place one half between two sheets of wax paper and roll to about ¼ inch thick.
Chill Dough: Place the rolled dough on a baking sheet and freeze for 10–15 minutes until firm but not rock hard. This helps the cookies keep sharp edges.
Cut Shapes: Cut pumpkins, ghosts, and bats. Place them on the prepared baking sheets, spaced slightly apart.
Bake Cookies: Bake for 11–14 minutes, until the edges are just beginning to turn light golden. Keep the centers pale for soft cookies.
Cool Completely: Let cookies cool 5 minutes on the pan, then transfer to cooling racks. Do not decorate until completely cool, at least several hours.
Make Royal Icing: In a mixing bowl, beat powdered sugar, meringue powder, and ¼ cup water for 2–3 minutes until glossy and thick. If desired, add a teaspoon of corn syrup for shine.
Adjust Consistency and Color: Divide icing into bowls. Add gel food coloring for white, orange, purple, and black. Thin some icing with extra water for “flood” consistency (flows and spreads smoothly), and keep some thicker for “detail” consistency (holds lines). Add more powdered sugar if it gets too thin.
Decorate Pumpkins: Outline the pumpkin with orange detail icing, leaving space in the middle. Flood the center with orange and smooth with a toothpick. After 10–15 minutes (slightly crusted), flood the side sections. Pipe a black stem. Once fully dry (about 8 hours), add eyes and a smile with an edible marker, plus pink luster dust cheeks if using.
Decorate Ghosts: Outline with white detail icing, then flood with white. Let dry fully (about 8 hours or overnight). Add eyes, a mouth, and a little “boo!” with an edible marker. Add rosy cheeks with luster dust if desired. For candy eyes, press them into the icing before it dries.
Decorate Bats: Outline the bat body with purple detail icing and flood. Let it crust for 10–15 minutes, then outline and flood the wings. After fully drying, add eyes and a mouth with an edible marker and highlight wing edges with purple detail icing. Add blush cheeks if desired.
Dry and Store: Let decorated cookies dry uncovered for at least 8 hours before stacking or storing. Keep in an airtight container up to 5 days, or freeze up to 3 months. For frozen cookies, let them come to room temperature inside the container before opening.