Heat a small sauce pan over medium high heat. Add 1 tablespoon of the butter. When the butter is hot but not burning, add the minced shallot.
Saute the shallot until translucent, about 2 minutes depending on the pan and your stove. What I mean by this is everybody uses different quality pans and different types of stove tops. Both factors will can have a great effect on how quickly something will cook so it is very difficult to give exact times.
Remove the pan from the stove and add the wine or Port. Return the pan to the stove and add some freshly ground pepper. How much you add will depend on your individual tastes.
Reduce the wine until there is just an essence left, practically nothing. If you are worried that I am giving alcohol to my girls, don't. It all burns off while you are reducing it. Most of it anyway.
When the wine is reduced, add the demi glace to the pan.
Simmer on low heat until the sauce is thick enough to coat a spoon or to a consistency you prefer. Some people have told me they like it less thick.
To finish the sauce and give it extra sheen and flavor, add the remaining tablespoon of butter. If you are watching your calories, you may want to skip this step.
Taste and adjust seasonings with salt & pepper.
Serve over your favorite steaks.