Preheat your oven to 350°F and butter a 9x13-inch baking dish well. I use real butter here because it prevents sticking better than cooking spray, and the edges get this beautiful golden color you actually want to see.

Combine 2 cups mashed sweet potatoes, 1/2 cup butter, 1 cup sugar, eggs, milk, vanilla, cinnamon, and nutmeg in a large bowl. Mix until completely smooth—I use a hand mixer for about 2 minutes to make sure the eggs are fully incorporated, because lumpy filling is disappointing.

Pour the sweet potato mixture into your prepared dish and smooth the top level. This is where patience matters: take 30 seconds to make it even, because it bakes more consistently that way.

Bake for 25 minutes until the filling is mostly set but still jiggles slightly in the center when you shake the pan gently. Don't skip checking at 23 minutes—ovens vary wildly, and you need to catch it at this exact moment.

Remove the casserole and sprinkle 1/2 cup pecans mixed with 1/4 cup brown sugar and 2 tbsp melted butter evenly across the top. This is the secret step: you're adding the pecan layer now, not at the beginning or end.

Top with 1 cup mini marshmallows, spreading them so they cover most of the surface without overlapping too much. I've learned that leaving tiny gaps actually helps them brown evenly instead of merging into one giant burnt layer.

Return to the oven for 22-25 minutes until the marshmallows are golden (not dark brown) and the filling barely jiggles when you move the dish. The marshmallows should look pillowy and slightly puffed, and they'll smell like toasted vanilla at this point.

Cool for 8-10 minutes before serving—this lets the filling set slightly so it doesn't fall apart when you scoop it out. I know this feels like forever when everyone's hungry, but those 10 minutes prevent the casserole from being completely runny.