Roughly chop the chocolate and add it to a small saucepan. Add a little of the milk until just covered and stir over medium heat until the chocolate has melted. Add mint simple syrup, cocoa powder, and a pinch of salt. While stirring, add the rest of the milk, followed by the corn starch. Stir until thick. Add the mint tea-infused Broken Shed Vodka to a teacup and top with the hot chocolate. Stir gently and then garnish with a mint sprig.