Heat milk in a medium saucepan until bubbles form on the edges (not boiling). Put cornstarch in a small bowl and whisk 1/4 cup of warm milk into it, making sure to get out all the clumps. This forms a cornstarch slurry, which will help thicken the hot chocolate. Pour the slurry into the saucepan with the rest of the milk and bring to a gentle simmer for about a minute. Turn off heat.