Line an 8 x 8-inch square tin with parchment, allowing a 1-inch overhang on all sides. Spray generously with nonstick spray.

In a large, heavy-bottomed stock pot, melt the butter over medium heat.

Add the marshmallows, stirring continuously until completely melted. Remove from heat and stir in the vanilla extract and fine sea salt.

Add the Lucky Charms cereal and stir until gooey and well combined, with no dry cereal remaining.

Pour into prepared tin. With oiled fingertips, press down* until evenly distributed and about 1 1/2 inches thick.

Refrigerate 30 minutes, or until set.

Melt the green candy melts (or white chocolate) in a microwave-safe bowl, stirring every 15-30 seconds, until completely melted. Scoop into a disposable piping bag.

Pipe melted green chocolate over cereal slab in lines or a zigzag pattern.

Decorate with additional cereal marshmallows, pressing down gently to secure. Sprinkle with pastel nonpareils.

Set aside and allow to cool at room temperature until chocolate sets, about 15 minutes.

Lifting by the parchment, transfer slab to a large chopping board. With a sharp, well-oiled chef’s knife, slice into squares. You can cut 3 rows x 3 rows (9 squares) for larger treats, or 4 rows x 4 rows (16 squares) for smaller treats.

Serve immediately. Store leftover treats in an airtight container at room temperature for up to 3 days.