4envelopes unflavored gelatin3 tablespoons plus 1 1/2 teaspoons
1 1/4cupslight corn syrup
1/3cupdark unsweetened cocoa powder
1Tbsppure vanilla extract
1cupconfectioners’ sugar and 2 Tbsp unsweetened cocoa for dusting
Line a 9×13 baking pan with plastic wrap leaving overhanging ends so you can easily lift out the marshmallows after they’ve firmed up. Brush or spray the wrap with oil so the marshmallows won’t stick.
In the bowl of a stand mixer fitted with the whisk attachment, put 3/4 cup of water. Sprinkle the gelatin over it and let it sit. .
In a medium saucepan, put the sugar, corn syrup, and salt. Begin heating it, stirring to dissolve the sugar. Then stop stirring and let the sugar syrup come up to 238 degrees. It will boil for a while before it gets there. Take off the heat and add the cocoa powder. Mix to combine well.
Then, with the mixer on low, slowly pour the hot syrup into the gelatin. Gradually increase the speed until it is on high. Beat on high for about 12 minutes until the mixture is stiff. Beat in the vanilla.
Pour the mixture into the prepared dish, smoothing it out evenly.
Let sit, uncovered, for 3 hours (or overnight)
Mix the remaining cup of confectioner's sugar with the 2 tablespoons of cocoa powder for dusting. Sift to combine well. Turn out the marshmallow block onto a surface covered with the sugar/cocoa mixture. Peel off the paper and dust the whole top surface with the sugar/cocoa as well.
Slice the marshmallows into squares using a sharp, oiled knife or pizza cutter. They will resist a bit, but keep at it. As you cut each strip, turn it to coat the cut ends with sugar/cocoa, and as you cut each individual marshmallow, do the same. All the sides should be well coated and not sticky.