Preheat oven to 350F.
Make a bain marie - add a small amount of water to a saucepan. Place a glass bowl in the saucepan, so that the bottom of the bowl skims the water. Heat the water over medium heat until it's simmering, not boiling.
Add the chocolate and butter to the bowl. Stir together with a spatula until melted, and set aside.
Separate the egg whites from the egg yolks.
Using a stand mixer fitted with the whisk attachment or an electric mixer, whisk the yolks and sugar together until light and creamy in color. Add the rum.
Add the melted chocolate mixture to the yolks. Continue mixing until well incorporated.
Add in the almond flour, and gently fold it in with a spatula.
In a bowl using an electric or stand mixer, whisk the egg whites until stiff peaks form.
Add the egg whites to the chocolate/almond mixture and gently fold them in using the spatula.
Butter and flour (with almond flour) an 8 inch cake pan. Pour the cake batter into the pan.
Bake for 30-35 minutes. You can test the cake with a toothpick to see if it's ready - but it doesn't need to come out completely clean, since the cake is fudgier in texture, the toothpick may not be perfectly clean.
Remove the cake from the pan after it has rested at room temperature for at least 40 minutes.
Dust with powdered sugar and/or cocoa powder.