Heat milk in saucepan over medium-low heat until you see steam. Turn off heat, add chocolate (do not stir), cover, and let sit for about 2 minutes or until chocolate melts. Stir until chocolate is thoroughly incorporated into milk. Set aside.

In a small custard cup, whisk cornstarch and liqueur together. (If you don't have a tiny whisk, put them in a small container with a lid and shake vigorously.) In a medium bowl, combine egg, brown rice syrup, and cornstarch mixture. Add 2 tablespoons of chocolate mixture to temper the eggs and mix well.

Add egg mixture to remaining chocolate mixture in saucepan. Cook over medium heat, stirring constantly, until mixture begins to thicken, about 3 to 4 minutes.

If you want a creamier texture, transfer mixture to food processor or blender (cap off since mixture is hot) and pulse for 20 seconds or so.

Place mousse in a bowl and cover with plastic wrap, pressing wrap directly onto the surface (to keep skin from forming). Refrigerate at least 2 hours.

Roll puff pastry to smooth it out. Using a heart-shaped cookie cutter, cut out at least 6 hearts. Place on prepared baking sheet and brush with beaten egg. Freeze until firm, about 5 minutes.

Bake hearts until puffed and golden brown, about 10 minutes. Transfer to rack and let cool.

Divide mousse among 6 ramekins or custard cups.

Top each with a puff pastry heart and whipped cream, if desired.