1. Place 2/3 of the chocolate in a large heatproof bowl. Bring a saucepan with 1 to 2 inches of water to a simmer; turn off. Set bowl of chocolate over saucepan; melt. Stir gently with a rubber spatula until it reaches 118 degrees on an instant-read thermometer. Remove bowl from saucepan; add remaining 1/3 chocolate to melted chocolate. Stir with a rubber spatula until melted chocolate cools to 84 degrees; remove unmelted pieces. Place bowl over pan; stir until thermometer reaches 88 degrees to 90 degrees.

2. Using a very small pastry brush, brush a streak of red cocoa butter on each half of four 2-part 2 1/2-inch sphere-shaped molds. Repeat process with white cocoa butter. Using a larger pastry brush, brush entire mold with chocolate. Transfer mold to freezer until chocolate is firm, 3 to 5 minutes. Carefully remove chocolate from molds, and place the bottom half of each mold onto a plate.

3. Fill a disposable pastry bag with mousse; cut tip off bag. Pipe mousse into the bottom half of each chocolate mold. Add 2 quince balls to mold and top with a tuile.

4. Working with one chocolate mold at a time, hold the top half of the mold, bottom side down, over a warm surface to soften slightly, about 10 seconds. Top bottom half of chocolate molds with top halves.

5. Pipe two small dots of mousse on the top of each mold; attach a heart tuile to each. Heat poaching liquid from quince and drizzle over chocolate molds; serve immediately.