In a stand mixer fitted with the paddle attachment, cream together the cocoa powder and butter on low speed until very smooth, about 2 minutes (scrape down the bowl with a rubber spatula as needed). Add the corn syrup, vanilla extract and salt, and continue to mix until evenly blended, 1 to 2 minutes. Add the hot water and mix on low speed until well combined. Add the confectioners’ sugar and mix on low speed. Once completely blended, increase the speed to medium and whip until the icing is very smooth and light, 2 to 3 minutes. (If you’d prefer the icing to be thicker once it is mixed, add more sifted confectioner’s sugar in ½ cup increments until you reach the desired consistency. Blend well between additions.) Use immediately or transfer to a storage container, cover tightly, and refrigerate for up to 1 week. Allow the icing to return to room temperature before using it. If necessary, beat the icing using the paddle attachment on low speed until it is smooth and spread able. Makes bout 5 cups, enough to fill and ice an 8- or 9-inch cake.