Add white chocolate to a microwave-safe bowl and microwave according to package directions. I microwave mine for 60 seconds at 50% power, twice, stirring between each session in the microwave. 

Add 1 spoon dollop of melted chocolate to a resealable sandwich bag and set aside. 

Add one dollop of white chocolate to an egg cup mold. Drag the chocolate up the sides of the mold so it's covered. Repeat until you have 3 cavities filled. (You will not use all of the chocolate) 

Freeze for 10 minutes. Remove from the freezer and add 1 tablespoon hot chocolate mix to each cavity. Top with marshmallows, making sure you keep them as level as possible or completely in the mold. 

If necessary, microwave remaining chocolate. Spoon a dollop of chocolate onto the top of the marshmallows and smooth evenly. Freeze for 15 minutes. 

Remove from freezer and pull away the sides of the mold from the hot chocolate bomb. Flip over the mold and peel it away from the bomb. Place on a flat surface. 

Microwave the small bag of chocolate for 15 seconds at 50% power. Knead the bag until completely melted and smooth. If necessary, return to the microwave for another heating cycle. Snip the tip of the bag and pipe a line of chocolate on top of the bomb and attach the horn and ears. Add the flowers and decorations. Finally, pipe white chocolate onto the top and sprinkle unicorn sprinkles on top. Let set to harden. 

To use, place the hot chocolate bomb into a mug and pour hot milk over it. Stir until completely dissolved.