How to Make It
Melt 1 tablespoon butter in a small saucepan over medium. Add shallot and garlic; cook, stirring often, until softened, 1 to 2 minutes. Remove from heat; let cool completely, about 10 minutes.
Stir together shallot mixture, demi-glace, and remaining 7 tablespoons butter in a bowl until combined. Spoon mixture onto a piece of parchment paper. Roll up into a 2-inch-thick log; twist ends of parchment to seal. Refrigerate until firm, about 2 hours.
Demi-glace is available online at amazon.com.