I start by adding the milk, sugar, black cocoa powder, and salt into a medium saucepan over medium-low heat. I whisk it slowly and constantly so nothing sticks or scorches on the bottom. It takes about 8 to 10 minutes for the sugar to dissolve fully and the mixture to get smooth and steamy with tiny bubbles forming around the edges.
Once the milk mixture is steaming, I add the chopped semisweet chocolate straight into the pan. I keep whisking until every piece has melted and the cocoa turns glossy and fully blended. Then I lower the heat and just keep it warm while I prepare the marshmallow topping.
I put the mini marshmallows into a microwave-safe bowl and microwave them in short 10 to 20 second bursts until they turn puffy and soft. I stir them with a fork right after so they smooth out, then let the mixture cool for a minute so it thickens slightly for drizzling.
This is my favorite part. I pour the hot cocoa into mugs first, then use a spoon to drizzle the melted marshmallow in thin concentric circles on top. With a toothpick or fork, I drag lines from the center outward to pull the circles into a spider web pattern.
I always serve this right away while it's hot and the marshmallow web still looks fresh and glossy. It only takes a minute to set, so I like to bring it straight to the table so everyone can admire the design before taking a sip.