Begin by melting your white melting chocolate. Place the chocolate in a microwave-safe bowl and heat in 30-second intervals, stirring in between, until fully melted and smooth.
Using a spoon, carefully coat the insides of your circle baking mold with the melted white chocolate. Make sure to cover the entire surface evenly.
Place the coated molds in the refrigerator for about 10-15 minutes to allow the chocolate to harden completely.
In a separate bowl, mix together the cocoa powder and granulated sugar until well combined.
Once the chocolate has hardened, carefully remove the shells from the molds. Fill half of the shells with the cocoa mixture and top with mini marshmallows.
To seal the cocoa bombs, warm a plate in the microwave for a few seconds. Take the empty half of the shell and gently press it onto the warm plate to melt the edges slightly, then place it atop the filled half. Press gently to seal.
Return the assembled cocoa bombs to the refrigerator for another 10 minutes to ensure they hold their shape.
To enjoy your White Chocolate Cocoa Bombs, simply place one in a mug and pour hot milk over it.