In a small bowl whisk together cocoa powder and powdered sugar.
Line an 8x8 inch pan with parchment paper or aluminum foil. Sift about ⅓ of the cocoa/sugar mix to coat the bottom of the pan.
In a medium saucepan add the sugar, corn syrup and ¼ cup of water. Stir to combine. Clip on a candy thermometer and heat without stirring to 238 degrees.
While the sugar is boiling, add remaining water to a mixing bowl and sprinkle on the gelatin.
When the sugar mixture reaches 238 degrees, turn on the mixer to low and slowly pour in over the gelatin. Beat for 8-10 minutes until fluffy.
While the marshmallows are fluffing, melt the chocolate in a small bowl in the microwave or over a double boiler.
Add chocolate and vanilla to the marshmallows at the end.
Pour mixture into prepared pan and sprinkle on ½ of the remaining cocoa/sugar mixture. Let sit for 3 hours.
Lift out the marshmallows using the foil/parchment paper. Cut to desired shape and size. Roll the cut edges in the remaining cocoa/sugar.