Heat a small pot filled with about 2 cups of water to a simmer. In a glass bowl that can fit into the small pan without falling in, mix the chocolate, butter, and coffee. Place the glass bowl with the chocolate over the pot of simmering water as shown in the picture above. Melt the chocolate over the simmering water, stirring constantly. When chocolate is fully melts and smooth, remove from heat and set aside, but keep the small pot with simmering water hot.
In a large wide bowl, put in ice and water to fill about half way.
In another glass bowl, mix the egg yolks, ⅔ cup sugar, rum, and water. Place glass bowl with egg mixture over the pot with simmering water. Heat the egg yolk mixture over the simmering water while whisking constantly. The mixture will thicken slightly. Do this for about 3-5 minutes. Remove from heat and immediately place bowl in the bowl of ice water (without getting any water into the yolk mixture). Whisk egg yolks until it has thickened and cooled. It will have a thickness similar to mayonnaise.
Fold the chocolate mixture into the eggs yolk mixture until fulling incorporated.
In another bowl, whisk eggs whites and salt until soft peaks form. Add sugar and vanilla and continue whisking until the eggs whites are thick, but just shy of forming stiff peaks.
Fold ⅓ of the egg whites into the chocolate mixture until fully incorporated. Gently fold in the the rest of the egg whites until just incorporated.
Transfer to a serving container as gently as possible and put into the refrigerator. Refrigerate until firm, about 4 hours.
Recipe by Learn by Cooking at https://www.learnbycooking.com/chocolate-mousse/