Set a small heavy-bottomed saucepan over medium-low heat and add the milk, espresso powder, vanilla paste, and kosher salt. Heat until scalding hot, about 8–12 minutes; do not let it boil.

While the milk heats up, finely chop the bittersweet chocolate and place it into a small bowl with brown sugar.

Once the milk is scalding, turn off the heat and add 1/4 cup of the hot milk into the bowl of chocolate. Whisk until completely smooth.

Transfer the milk-chocolate mixture back into the pot and whisk until fully melted and homogeneous.

Return the pot to medium-low heat, whisking frequently for 1–2 minutes until thick enough to coat the back of a spoon.

Taste and adjust seasonings as needed. If too thick, whisk in a splash of milk; if not sweet enough, add more brown sugar; if cloying, add a bit more salt.

Divide amongst two mugs and serve immediately with whipped cream, if desired.

Pour the chilled heavy cream into a small bowl. Add vanilla paste and sugar if desired. Whisk until soft peaks form, then spoon on top of the hot chocolate.