Make the cake glaze before baking the cake layers. In a large saucepan, melt ¾ cup of the butter and mix it with the melted chocolate and 4½ cups of the sugar. Stir in the evaporated milk, 2 teaspoons of the vanilla, and the instant coffee. Cook the glaze over medium-high heat until it boils. Reduce the heat to low and continue cooking, stirring constantly, until the mixture thickens, about 20 minutes. Remove the glaze from the heat. When cooled a bit, return to low heat as needed, as the glaze must be warm to spread on the cake layers.