Keep the unsalted butter near 68°F, then beat it with granulated cane sugar and light brown sugar for 2 to 3 minutes until the mixture looks pale and slightly fluffy. Add the large egg and pure vanilla extract, then mix for 45 to 60 seconds and scrape the bowl once halfway through. Stop as soon as the batter looks smooth. Overbeating after the egg can make the cookies spread more than you want. You'll see the sugar crystals become less obvious and the mixture will smell strongly of vanilla and butter. It should look creamy rather than greasy. Purple and Orange Halloween Cookies need this even base so both colored doughs bake at the same rate. Cream only until light and smooth because trapped air controls the final spread.

Whisk unbleached all-purpose flour with baking powder and fine sea salt, then add it to the butter mixture at about 70°F. Fold with a rubber spatula for 30 to 45 seconds until no dry pockets remain. Divide the dough evenly and work 1 teaspoon purple gel food coloring into one half and 1 teaspoon orange gel food coloring into the other for about 60 seconds each. Stop there. The colors should look mostly even. The dough should feel soft but not sticky and each half should show a few natural streaks rather than one flat artificial tone. You'll notice the floury smell disappear as the dough comes together. Mix too long and the cookies can turn firm instead of tender. Fold gently because limited gluten development keeps the crumb soft.

Pinch off 1 tablespoon of purple dough and 1 tablespoon of orange dough for each cookie, press the pieces together and twist once before rolling lightly into a rough ball. Make 12 cookies and press each to about 1/2 inch thick, then add Halloween sprinkles with a light fingertip press. Chill the shaped dough at 38°F for 20 minutes. Skip the chill and the color pattern can blur as the butter softens. The balls should look marbled rather than perfectly split, with irregular purple and orange seams running across each surface. Some sprinkles will sit deeper than others. That's good. (My first batch stayed at room temperature too long. The swirls spread into wide flat cookies.) Chill before baking because firm fat delays spread long enough for the edges to set.

Bake the chilled cookies in a 350°F preheated oven for 10 to 12 minutes on the center rack, keeping the 12 rounds spaced about 2 inches apart. Check early. At minute 9, check the front row and rotate the tray 180 degrees only if your oven browns unevenly. Pull the tray as soon as the edges look set and the centers no longer appear wet. Go past 12 minutes and the bottoms can darken before the centers stay tender. The orange sections deepen slightly and the purple stays bold while the tops develop tiny cracks around the marbled seams. You'll smell toasted butter and vanilla right before they're ready. Purple and Orange Halloween Cookies should still feel soft in the center when they leave the oven. Bake at 350°F because moderate heat sets the edges before the centers dry out.

Leave the cookies on the hot tray for 10 minutes, then transfer them to a cooling rack for another 20 minutes at about 70°F before serving. Move them with a thin spatula while the centers are still slightly flexible, and let them finish setting before stacking or plating. They'll firm as they cool. Move them too early and the soft centers can bend or crack around the marbled seams. Purple and Orange Halloween Cookies should have lightly crisp edges and soft centers with nonuniform purple and orange patches across every round. A few sprinkles may sink or tilt, which keeps the batch looking homemade. Plate one cookie once it's fully cool. Cool for 30 minutes total because residual heat finishes the center without overbaking it.