Melt the unsalted butter over medium heat, about 5 on a 10-dial, and keep it there for 5 to 7 minutes. Swirl every 20 to 30 seconds so the milk solids move across the bottom instead of collecting in one spot. The foam will rise first and then thin out as small golden flecks appear. Take the butter off the heat once those flecks turn amber and the liquid smells like toasted hazelnuts. Go much darker and the milk solids can taste bitter. Then let the browned butter cool for 15 to 20 minutes until it reaches about 85°F to 90°F. It should stay fluid but shouldn't feel hot. Cool it before mixing because hot butter melts the sugar too fast.

Combine the cooled browned butter with dark brown sugar and granulated cane sugar at about 70°F room temperature. Mix firmly for 45 to 60 seconds until the mixture looks glossy and slightly thick. Then work in the large egg and pure vanilla extract for another 30 seconds. Add all-purpose flour, baking soda and Diamond Crystal kosher salt. Fold from the bottom upward until only a few dry flour streaks remain, then stop. Overmix here and Brown Butter Halloween Candy Cookies lose their tender centers. The dough should look thick and slightly tacky with dark butter flecks running through it. No loose flour should remain. Mix only until combined because extra stirring builds more gluten and makes the cookies tougher.

Fold 3/4 cup Halloween milk chocolate M&M's and 1/2 cup chopped mini peanut butter cups into the dough using broad turns that lift dough from the bottom. Stop after about 8 to 10 folds so the candy stays in visible pockets instead of breaking apart. Cover and chill the dough at 40°F for 30 minutes. Then divide it into 16 rough portions of about 2 tablespoons each and leave the tops uneven. Skip the chill and the warm fat can make Brown Butter Halloween Candy Cookies spread too thin. The chilled portions should feel firm enough to lift without sticking badly, but they shouldn't feel hard. You'll still see bright candy pieces on the rough surfaces. (I once skipped the 30 minute chill. That batch spread into one nearly solid sheet.) Chill the dough so the butter firms before oven heat hits it.

Arrange 8 dough portions on each tray with about 2 1/2 inches between them. Bake one tray at a time at 350°F for 10 to 12 minutes and rotate the tray once at minute 7. The cookies will spread into uneven rounds while the edges deepen from tan to golden brown. Pull them out when the outer half inch looks set but the middle still appears soft and slightly puffed. Go beyond 12 minutes and the centers lose much of their chew. You'll smell toasted butter and warm chocolate near minute 9. The candy shells may crack and the peanut butter cup pieces will look glossy. Bake Brown Butter Halloween Candy Cookies only until the edges set because carryover heat finishes the soft centers.

Leave the baked cookies on the tray for exactly 10 minutes at normal room temperature, about 68°F to 72°F. During that rest the soft centers settle and the edges become firmer without turning hard. Then move each cookie carefully and let it cool for another 15 minutes before serving. Don't stack them while warm because the softened chocolate and peanut butter candy will smear. This final rest gives Brown Butter Halloween Candy Cookies their chewy middle instead of a fragile underbaked texture. The finished cookies should have crisp golden rims and wrinkled tan centers with small glossy chocolate pockets. Break one open and the middle should bend before it snaps. Cool before serving because the starches and melted sugars need time to set.