In a large glass measuring cup or bowl, melt the chocolate for 15 seconds; gently stir. Repeat using 15 second intervals and stirring well, about 1-½ minutes or until chocolate is chocolate is smooth and the internal temperature is 105°F for milk or white chocolate and 120°F for dark or bitter-sweet chocolate. Let stand until temperature reaches 90°F.
Pour about 1 tablespoon of chocolate into the mold. Using a small spatula, pastry brush, or back of a spoon, spread the melted chocolate up the sides of the molds. .
Place the molds in the refrigerator for 15 minutes or let stand at a cool (70 degree) room temperature for 30 minutes, or until set.
Remove the chocolate from the molds. Turn the molds upside down onto a piece of parchment paper and gently push the chocolate sphere out.
Arrange the molds, hollow side up, on the parchment paper. Add 1 rounded tablespoon of hot cocoa powder to the inside of one-half of the spheres. Add 5 to 6 mini marshmallows on top of the cocoa powder.
Add approximately ½ cup of the melted chocolate to a quart-size storage bag and push the chocolate down into one corner to make a cone shape. Using a scissors, snip a tiny corner of the bag open. Pipe a line of chocolate around the top edge of the filled sphere. Place an empty sphere on top and gently press together. You can use your gloved finger to smooth any excess chocolate around the edges.
Steam a cup of milk. Place chocolate bomb into the bottom of a mug and pour the milk over the top. Stir gently and watch the surprised unfold.