Line a 9x5 inch loaf pan with 2 layers of parchment paper, letting each layer hang over 2 sides of the pan (this will make it easy to lift out the frozen whipped cream).
In a large bowl, whisk together the cream, sugar, and Grand Marnier (if using), until the cream reaches a soft peak. Use a spatula to spread the whipped cream into the loaf pan, taking care to even out the top. If the parchment paper starts to fold over, dab a small amount of whipped cream between the paper and sides of the pan to help hold it in place. Freeze until solid. (Since this will take a few hours, I recommend freezing overnight if you have time. Cover the pan in plastic wrap to prevent odors from seeping in).
Once the whipped cream has frozen, remove the loaf pan from the freezer. Use the parchment paper to gently lift the cream onto a cutting board (if there’s resistance, allow it to thaw for 1-2 minutes first).
Using a heart-shaped cookie cutter (use one that works with the size of your mugs), press firmly into the cream and lift straight up, then gently tap the cookie cutter onto a clean cutting board to release the heart. Do this several times (the yield will depend on the size of the cookie cutter; you can definitely get 4-6 hearts even with a larger cutter). Place the cutting board with the frozen hearts back in the freezer while preparing the hot chocolate.
Place the semi-sweet chocolate in a medium-sized bowl, preferably one with a pouring spout. Set aside.
In a small saucepan, heat the milk and vanilla over medium-high heat. Once the milk reaches a simmer, ladle some into the chocolate and stir with a spatula to melt. Continue adding hot milk and stirring until the hot chocolate is smooth.
Pour the hot chocolate into 2 mugs and top each with a whipped cream heart.
You can use any hot chocolate recipe with the whipped cream hearts. I kept it very simple since those take a bit of effort! Or you can enjoy this hot chocolate on its own, with regular whipped cream, or marshmallows. You can also reduce the calorie content by using lowfat/nonfat milk in the hot chocolate (but that won't be as tasty).