Preheat the oven to 425°F. Lightly butter four 6-ounce ramekins or custard cups. Bring a few cups of water to a boil in a medium saucepan over low heat. Place a glass mixing bowl over the pan (but not touching the water) and add the chocolate and butter. Cook, stirring occasionally until both the chocolate and butter have fully melted. Keep warm. Use an electric mixer to beat the eggs, egg yolks, sugar, and salt until pale yellow and thick, about 5 minutes. Stir in the melted chocolate mixture, the flour, vanilla, and instant coffee if using. Pour the mixture into the prepared ramekins. Stick one good chunk of chocolate in the center of each ramekin. Bake the cakes on the center rack for 8 to 10 minutes, until the exterior is just set (the center should still be mostly liquid). The cakes can be eaten straight from the ramekins, but it's more dramatic to slide them on to plates (after letting them rest for a minute or two), where the molten chocolate can flow freely.