In two small, microwave-safe bowls, melt semi-sweet and white chocolate chips in 30-second increments stirring in between. It should take about 60 to 90 seconds.
Spoon about 1 tablespoon of each into each half-sphere mold. Using the back of a spoon or a pastry brush, gently swirl the chocolate together for a marbled effect and spread to coat the sides. Don’t swirl more than 4 times in each mold as the colors will mix and be indistinguishable. Let set in the fridge or the freezer for 10 to 2 minutes or until hardened.
Once hardened, reinforce the edges with a little of the remaining chocolate. Let set until hardened.
Carefully remove each half-sphere from the molds. Be sure to handle each half-sphere very little as the chocolate will melt from the warmth of your hands and fingerprints will be visible. Use nitrile gloves to reduce melting.
In 6 of the half spheres, spoon 2 tablespoons of the hot cocoa powder. Top with 1 teaspoon of espresso powder and mini marshmallows.
Warm a small plate in the microwave for 45 seconds. Place an empty half-sphere on the plate for a few seconds until the edge begins to melt. Place empty half onto the corresponding filled half-sphere ensuring that the edges are aligned.
Let sit for a minute to harden before handling. Repeat with the remaining 5 spheres until all of the cocoa bombs are assembled.
To make hot cocoa, place a cocoa bomb into a mug and top with 8 ounces of hot milk. Once the shell has melted, stir to emulsify. Serve immediately and enjoy!