Pour the whole milk and heavy cream into the crockpot, stirring briefly to combine.

Sprinkle the cocoa powder evenly over the liquid, followed by the sugar and salt. Whisk gently until the cocoa powder is mostly dissolved and no dry pockets remain.

Add both the semi-sweet and milk chocolate chips, making sure they are fully submerged in the liquid.

Cover the slow cooker with the lid and set it to low heat. Allow the mixture to cook for 2 to 2½ hours, stirring every 30 minutes to help the chocolate melt evenly and prevent scorching along the edges.

Once the chocolate chips are completely melted and the mixture looks smooth and glossy, stir in the vanilla extract.

Continue cooking for another 15–20 minutes, stirring once more, until the hot chocolate is fully unified, thickened slightly, and steaming but not boiling.

Taste and adjust sweetness if needed by stirring in a small amount of sugar, allowing it to dissolve fully before serving.

Switch the crockpot to the warm setting to maintain the perfect serving temperature for up to 2 hours, stirring occasionally to keep the texture silky.