Line the countertop or a large tray with a silicone mat. Position foil cornucopia template (see notes) lengthwise in the center of the mat.

Melt the butter in a large, heavy-bottomed saucepan or stock pot.

Add the marshmallows, stirring continuously until completely melted.

Remove from heat. Stir in the vanilla extract and fine sea salt.

Stir in the puffed rice cereal. Continue stirring until well combined, with no dry cereal remaining.

With greased hands, shape cereal mixture around the foil template. (See notes.)

Allow to cool overnight, uncovered, at room temperature. Cereal mixture will firm up as it sets.

Carefully remove the funnel. Position cornucopia sideways on a plate or serving platter, or in the middle of your table.

Fill with candy. Serve immediately, or cover with greased aluminum foil until ready to serve. Store leftovers at room temperature for up to 2 days.