In the bottom of a cold crockpot, whisk sweetened condensed milk with cocoa powder and vanilla until smooth.: When you begin, you will notice the cocoa smells rich and slightly toasted as it hits the sweetened condensed milk, and whisking right in the cold pot helps dissolve dry bits before adding the larger volume of milk. The aroma of cocoa opening up against the sweet, milky scent tells you the base is coming together. A common mistake is not breaking up the cocoa lumps early, which leaves gritty pockets later, so whisk until you can see a glossy paste and no dry streaks remain. If you skim a little mixture off the whisk and rub it between your fingers, it should feel smooth not grainy. Using a firm whisk and pressing against the pot bottom releases trapped clumps and speeds the process. Keep the vanilla measured and add it here so it disperses evenly, brightening the chocolate while it is still in concentrated form.
Whisk in milk, half & half and creamer, being sure to get to the edges. Sprinkle in the salt. Cover and warm on high for 120 minutes or low for 240. You're not trying to cook this, just warm it through! Once it's to the desired temperature, switch crockpot to 'warm'.: As you pour the whole milk , half & half , and creamer into the chocolate paste, you will hear a soft settling sound as the liquids integrate and the scent becomes rounder. Gently whisk toward the edges to capture any paste clinging at the perimeter, because trapped chocolate will cause uneven texture. The salt, even in a small amount, will change the flavor immediately, lifting the chocolate notes. When the crockpot is covered and warming, you should see a gentle, barely perceptible steam and occasional small bubbles near the sides rather than a vigorous boil. This slow heat melds flavors without breaking dairy. A frequent error is turning the heat too high, which risks scalding and a cooked flavor, so follow the timing and aim for steady warmth. If you notice a skin forming on top, stir more frequently to reincorporate it. I find switching to 'warm' once the temperature is right keeps the drink ready without over reducing it.
When you're ready to serve, add in mini marshmallows and more coffee creamer if you so desire {which you totally should!!} Enjoy!: The moment you hit serving temperature, the kitchen will smell deeply chocolatey and the surface will be smooth with a subtle sheen. Leaving it on high continues to concentrate and can alter texture, so moving to 'warm' preserves the velvety mouthfeel and prevents a film from forming. If you let it stay too long on high, dairy can tighten and the flavor shifts; a common issue is overheating which leads to a dull finish. Stir occasionally on warm to keep the surface consistent and check sweetness, adjusting with a splash more creamer if you want a silkier cup.
When you're ready to serve, add in mini marshmallows and more coffee creamer if you so desire: Adding mini marshmallows right at service gives a soft melting top that looks inviting and changes the initial mouthfeel as you sip. If you pour the hot chocolate into mugs and then add marshmallows, they will float and slowly soften, releasing a toasted sugar scent as they warm. A nice trick is to pour a small splash of the creamer into each cup for a glossy finish and a flavor lift. One mistake is adding marshmallows too early in the crockpot, which makes them dissolve completely and removes the textural contrast, so reserve them for the moment of serving. Watch for bubbles as you ladle, and aim to create a gentle pour to keep the marshmallows atop the cup for presentation.