Place 2 silicone molds (each with about 15 squares, approximately 1.5in x 1.5in x 1.5in per square) onto a rimmed baking sheet. Set aside.

In a large bowl, combine the semi-sweet chocolate chips and the broken pieces of the bittersweet chocolate baking bar. Set aside.

In a medium saucepan, over medium heat, add the sweetened condensed milk and heavy cream. Heat the mixture until it reaches approximately 120-130°F, which will take about 3-4 minutes.

Pour the hot milk mixture over the chocolates in the large bowl, ensuring the chocolates are completely covered. Allow to sit undisturbed for 3-4 minutes before whisking until fully incorporated and smooth.

Transfer the melted chocolate mixture into a large piping bag or a zip-top plastic bag with one corner snipped off. Keep the open tip facing up until ready to pipe.

Pipe the melted chocolate into each square opening of the silicone molds, filling them about ¾ full. Leave some space to top with 4-5 mini marshmallows, gently pressing them down. If omitting marshmallows, fill squares completely.

Insert one lollipop stick into the center of each chocolate square. Place the baking sheet with the molds into the refrigerator to chill and firm up for 3 hours, or preferably overnight.

Once the hot chocolate squares are firm, remove two individual squares from the silicone mold to make one mug of hot chocolate. (This recipe yields 14-15 servings, using 2 squares per 1 cup of milk).

To prepare the hot chocolate drink, place two chocolate cubes on a stick into a large 10-12 oz. mug.

In a microwave-safe 2-cup measuring cup with a spout, heat the whole milk on high for 2-2.5 minutes until scalded (hot but not boiling). Pour the hot milk over the chocolate cubes in the mug. Stir with the lollipop sticks until the chocolate is completely melted, creating a rich cup of hot chocolate with marshmallows. Discard the lollipop sticks after stirring.