Line a quarter-sheet pan or 9-by-13-inch pan with parchment, pressing the parchment into the corners and leaving an overhang on two sides for easy removal. Spread the 3 tablespoons (45 g) puffed quinoa or tiny puffed rice in an even layer on the bottom of the lined pan.

Place about 3/4 of the 8 oz (226 g) finely chopped dark chocolate in a heatproof bowl. Melt gently using a microwave in short bursts (stirring between bursts) or over a double boiler—use very low heat and avoid overheating.

If using the optional cocoa butter (1 to 2 teaspoons / 2 g), add it while the dark chocolate is melting to improve fluidity. Remove the bowl from heat when the chocolate is mostly melted, then stir in the remaining dark chocolate pieces until fully melted and the mixture has cooled and thickened slightly (about 88–90°F / 31–32°C if you have a thermometer).

Pour the melted dark chocolate over the puffed quinoa and spread into an even layer, working almost to the edges. Tap the pan firmly a few times on the countertop to release air bubbles.

Let the dark chocolate layer set at cool room temperature until fully firm, about 15–20 minutes. If it does not set at room temperature, chill briefly in the refrigerator until firm, then allow it to come back to room temperature before proceeding.

Place about 3/4 of the 12 oz (340 g) finely chopped white chocolate in a clean heatproof bowl. Melt gently (microwave in short bursts or over a double boiler), remove from heat when mostly melted, and stir in the remaining white chocolate until smooth and fully melted.

Spoon about one third of the melted white chocolate into a separate small bowl. Sift the 1 teaspoon (4 g) matcha powder over the white chocolate in that bowl and stir until evenly combined.

Spread the remaining two thirds of the melted white chocolate over the set dark chocolate layer, leaving a small border if needed.

Drop spoonfuls of the matcha-white chocolate mixture over the white chocolate layer. Use a toothpick or skewer to drag through the white and matcha blobs to create swirls or a marbled pattern.

Sprinkle with optional sprinkles or other decorations as desired.

Allow the bark to set uncovered at cool room temperature until firm, about 1–2 hours. If your chocolate was not tempered or your kitchen is warm, refrigerate to set; if you refrigerate, let the bark come back to room temperature briefly before serving for best texture.

Lift the parchment from the pan, then break or cut the bark into pieces. Store at room temperature in a cool, dry place, or refrigerate if the chocolate is untempered.