Prep the pan: Line a 9x13 inch pan with parchment paper, leaving overhang on two sides. Lightly grease the parchment with butter or cooking spray.

Melt the butter: In a large pot over low heat, melt the butter completely until it bubbles gently at the edges, about 2 minutes.

Melt the marshmallows: Add the mini marshmallows to the pot and stir constantly with a wooden spoon until fully melted and smooth, about 3 to 4 minutes. Keep the heat low so the mixture doesn't scorch.

Stir in vanilla: Remove the pot from heat and stir in the vanilla extract until combined.

Fold in the cereal: Add the rice cereal to the pot in two batches, folding gently with a spatula until every piece is coated in marshmallow.

Press into the pan: Transfer the mixture to the prepared pan. Using a greased spatula or lightly buttered hands, press it evenly and gently into the corners, about 1 inch thick.

Add candy while warm: Scatter candy corn and sprinkles over the top while the mixture is still warm, pressing lightly so they stick to the surface.

Drizzle candy melts: If using, drizzle the melted orange candy over the top in thin lines for a decorative finish.

Cool completely: Let the pan sit at room temperature for about 20 minutes until fully set and firm to the touch.

Slice and serve: Lift the slab out using the parchment overhang, then cut into 16 squares with a sharp knife for clean edges.