Preheat the oven to 375F and line a 9” cake pan with parchment paper and butter.
Melt butter and chocolate in the microwave in 20s increments for the cake.
While the chocolate is melting, whisk eggs and sugar for 5-7 minutes until thick, ribbony, and light in color.
Pour the melted butter and chocolate into the egg mixture and fold together.
Sift in cocoa powder and salt, then gently fold. Add the coffee and fold again.
Pour the batter into the pan and bake for 20-25 minutes until a toothpick comes out with moist crumbs attached.
While the cake is baking, prepare the sauce. Melt butter and chocolate, whisk egg and sugar, and mix the two batters similarly to the cake. Heat over medium-low heat until the mixture thickens. Add cream and cook until you get a sauce-like consistency (it will thicken more as it cools).
Transfer the cake to a stand and pour over the sauce.
Enjoy it as is or with a scoop of vanilla ice cream!