Making the Cointreau custard :
With a handheld mixer, whisk the egg yolks with the sugar in a small stainless steel (or heat-proof) mixing bowl until you get a pale yellow foam.
Prepare a double boiler: Fill a saucepan with water, making sure the water barely covers the bottom of the stainless steel mixing bowl. I always like to add a little kitchen towel on top of the saucepan. The bowl won't jiggle and there won't be any splatter of water in your egg mixture. Bring the water to a boil, then lower the heat to a gentle simmer. Place the stainless steel bowl over the saucepan and, using the handheld mixer, start whisking the egg mixture vigorously for about 3 minutes to thicken the texture of the egg yolks. The mixture should fall like a ribbon of sauce when you lift the whisk. Add 2 tablespoons Cointreau. Stir constantly using a silicone spatula. The mixture will thicken. Remove the saucepan from the stove. Add a pinch of salt and stir well.
Note: A way to know that the custard is ready (so the egg yolks are fully cooked) is to use a digital thermometer and wait until the temperature reaches 160-175°F.
Reserve about ½ cup custard for garnish.
Making strawberry Cointreau mousse:
In another mixing bowl, whisk the mascarpone cheese with 2 tablespoons of powdered sugar to soften it. Slowly add the custard and melted white chocolate to the mascarpone cheese, adding just a little custard at a time. Add 1 teaspoon strawberry extract (if used).
Clean your handheld mixer in hot soapy water. Always make sure the beater blades are completely clean prior to whipping cream for optimum results. Rinse with cold water to ensure the blades are cold. The whipping cream should also be whipped cold. Whip 2 cups of cream (at low speed) for 2-3 minutes until creamy and smooth. Add the rest of the powdered sugar. Increase to maximum speed and keep beating for another 2-3 minutes, until it forms soft peaks.
Pour ⅓ of the whipped cream into the Cointreau mixture. Mix well using a silicone spatula. Pour in the rest of the whipped cream; gently fold the whipped cream to get a light and airy mousse.
For the strawberry chocolate Cointreau sauce:
Place the bittersweet chocolate, raw sugar and the corn syrup and a pinch of salt in a bowl.
In a little saucepan, bring the rest of the heavy cream to a boil and pour over the bittersweet chocolate. Add 1 teaspoon strawberry extract and Cointreau to the dark chocolate. Mix with a wooden spoon. The sauce will thicken as the chocolate melts. Set aside.
Transfer the Cointreau mousse to a piping bag.
Create 16 small rounds of shortcake that fit the dessert cups, using a circle cutter.
Spoon about 1½ teaspoons custard into each serving cup. Drizzle a bit of the chocolate sauce (make sure it's visible on the wall of the cups), then pipe the mousse to fill the cups entirely. Plastic wrap, seal and chill them in the refrigerator for at least 4 hours.
When you're ready to serve, place each Cointreau mousse cup on an individual serving plate and top with a fresh blueberry (as a reminder of the flavoring in the chocolate sauce) and decorate with a sprinkle of chopped almonds.
Cointreau is a brand of orange-flavored triple sec liqueur. You could substitute 2 teaspoons vanilla extract for the Cointreau to make the dessert kid-friendly.
If you don't have cute cup props to serve the mousse in, you could serve the dessert in a large triffle serving mold.
The use of light corn syrup helps the chocolate sauce maintain a beautiful texture, even once it cools down.