Prepare the Cherry Compote:

1. Cook the Cherries: In a medium saucepan, combine the cherries, sugar, and lemon juice over medium heat. Cook, stirring occasionally, until the cherries release their juices and soften, about 5-7 minutes.
2. Thicken the Compote: If desired, mix cornstarch with a little water and add it to the cherry mixture to thicken. Continue to cook for another minute, then remove from heat.
3. Cool the Compote: Stir in the vanilla extract, then let the cherry compote cool to room temperature. Once cooled, set aside or refrigerate until ready to use.

Prepare the Chocolate Mousse:

4. Melt the Chocolate: In a small saucepan, heat the heavy cream over medium heat until it just begins to simmer. Pour the hot cream over the chocolate chips in a heatproof bowl and stir until the chocolate is fully melted and smooth.
5. Add Mascarpone (Optional): For an ultra-creamy texture, stir in the mascarpone cheese at this stage until fully combined.
6. Cool the Mousse Base: Let the chocolate mixture cool slightly, about 10 minutes. Then, fold in the vanilla extract.
7. Whip the Mousse: In a separate bowl, whip the remaining 1 cup of heavy cream and sugar until stiff peaks form. Gently fold the whipped cream into the cooled chocolate mixture until smooth and combined.

Assemble the Mousse Cups:

8. Layer the Mousse: Spoon or pipe the chocolate mousse into individual cups or glasses, filling each about halfway.
9. Add the Cherry Compote: Spoon the cooled cherry compote over the mousse layer, creating a bright, fruity topping.
10. Top with Whipped Cream: In a separate bowl, whip the cream with powdered sugar and vanilla extract until stiff peaks form. Spoon or pipe the whipped cream on top of the cherry layer.
11. Chill the Mousse Cups: Place the mousse cups in the refrigerator for at least 2 hours to allow the flavors to meld and the mousse to set.
12. Serve: Garnish with extra cherries, chocolate shavings, or a mint leaf if desired before serving.