Preheat an oven to 350 degrees F (175 degrees C). Line a muffin pan with paper or foil liners.
Melt the butter and chocolate in the top of a double boiler over barely simmering water, stirring frequently and scraping down the sides with a rubber spatula to avoid scorching. Remove from heat and let cool.
Blend the eggs and sugar with a mixer in a large bowl. Stir the cooled melted chocolate into the egg mixture. Add the vanilla extract and instant coffee. Sift in self-rising flour and stir until well combined. Fill muffin cups 2/3 full. Bake in the preheated oven until the tops spring back when lightly pressed, about 15 to 18 minutes.
