Melt 10 oz of the chocolate in 30-second microwave bursts, stirring until smooth.
Spoon into 12 silicone half-sphere molds, swirling to coat. Chill 10 minutes; add a second thin coat and chill again.
Pop the shells out of the molds. Fill 6 halves with 1 tbsp hot cocoa mix and a spoonful of mini marshmallows each.
Warm a plate in the microwave, press an empty half rim-down on it for 2 seconds to melt the edge, then seal it onto a filled half. Repeat for all 6.
Melt the remaining 2 oz chocolate, drizzle over the bombs, and add sprinkles. Chill 5 minutes to set.
To serve: drop one bomb into a mug and pour 1 cup hot milk over it. Watch it melt open, then stir and sip!