For the Marshmallows:
Combine remaining 1/2 cup water, sugar, corn syrup, and salt in a medium heavy saucepan over medium-high heat; bring to a boil, stirring occasionally. Cook, without stirring, until a candy thermometer registers 250°. Pour sugar mixture into the bowl of a stand mixer; let stand until a candy thermometer registers 210°.
Add 1 teaspoon vanilla. Increase speed to high; whip mixture at high speed until light and fluffy (about 5 minutes). Reduce mixer to medium speed, and gradually add 1/4 cup cocoa; beat until combined.
Drizzle melted chocolate over marshmallows; let stand until chocolate is set. I added an additional 2 Tbsp cocoa to the 1/4 cup (so 6 Tbsp total) then I also adjusted the coating to be 1/4 cup powdered sugar, 1/4 cup cornstarch and 2 Tbsp cocoa powder.