Place silicone molds onto a rimmed baking sheet.

In a large bowl, combine semi-sweet chocolate chips and bittersweet baking bar.

Heat sweetened condensed milk and heavy cream in a saucepan until 120-130°F.

Pour hot milk mixture over chocolates and let sit for 3-4 minutes, then whisk until smooth.

Transfer melted chocolate to piping bag and pipe into molds, leaving space for marshmallows.

Press 4-5 mini marshmallows into each chocolate square, then insert a lollipop stick.

Chill in refrigerator for 3 hours or overnight.

Remove two squares per mug, place in hot milk, and stir until melted.