In a microwave-safe dish, heat 1 cup of chocolate for 30-second intervals until melted, stirring after each interval.
Use a spoon to paint the inside of the spheres with the melted chocolate. Take care to paint chocolate all the way to the edge of the mold. Gravity may pull the chocolate from the top edges of the mold, so check after coating all the half-spheres to ensure the chocolate reaches the edge. If there are thin spots of chocolate, recoat them because thin spots can easily break. You do not have to grease the mold.
When all of the half-spheres are "painted" with chocolate, place the mold in the refrigerator to harden the chocolate. This will take 5-10 minutes or longer.
When the chocolate hardens, remove it by peeling the mold away from the chocolate. Then press the bottom of the mold to release the chocolate half sphere, being careful not to break the chocolate.
Add 2 tablespoons of hot chocolate powder and 2 tablespoons of marshmallows to half the spheres.
Heat a non-stick frying pan until it is warm enough to melt chocolate, but not too hot. Place an empty half-sphere in the pan, rim-side down, until the chocolate edge starts to melt. This will only take a couple of seconds. Remove the half-sphere from the pan and place it on a half-sphere already filled with hot chocolate. The melted edge will act as chocolate glue, sealing the sphere. Add leftover melted chocolate from the pan to seal holes around the edge. Wipe off any extra chocolate.
Melt the white chocolate in the microwave for 30-second intervals, stirring after each interval. Add pink gel food coloring to the chocolate until the desired color is achieved. Add a few drops of vegetable oil to the chocolate to thin it for piping. Place the chocolate in a plastic resealable sandwich bag and snip off a small corner for piping.
Using a back and forth motion, pipe stripes onto 1 ball one at a time. Repeat for all the balls.
To serve, place a ball in a large mug. Slowly pour 1 1/4 to 1 1/2 cups of hot milk on top of the ball, melting it with the milk. If the sphere doesn't fully melt and the marshmallows pop out, use a spoon to rotate the ball to melt it further, allowing the marshmallows to pop out of the ball.