In a medium-sized mixing bowl or the bowl of a stand mixer, mix butter, molasses, and sugar until fully combined.
Melt the chocolate in the microwave or in a double boiler.
Once the melted chocolate is cool enough to touch, add it to the butter mixture and mix until just combined.
Mix in egg and egg yolk until just combined.
Mix in vanilla and sour cream until just combined.
Sift together flour, salt, baking soda, and cocoa powder into a separate medium-sized bowl. Use a fork or a whisk to ensure that the ingredients are mixed well after sifting.
Add dry ingredients to wet ingredients in three additions, mixing until just combined after each addition.
Mix in rum until just combined.
Fill mini cupcake liners 2/3 full and put in the oven.
Check for doneness at 12 minutes - a toothpick inserted into the center of a cupcake should come out clean. If they are not done, check again every three minutes until they are done.
Immediately remove cupcakes from the cupcake tin and place on a cooling rack or on the counter to cool to room temperature.
Pipe with chocolate mousse frosting.
Optionally, roll the bottom edge of the frosting in gold nonpareils.