Heat butter in a medium saucepan over medium heat until bubbling. Crush garlic with the side of a heavy knife. Add garlic, onion and carrot to saucepan. Cook, stirring, until onion is translucent, about 4 minutes. Add stock and maple syrup, then increase heat to high and boil 10 minutes, or until liquid is reduced to about 1/2 cup. Add beer, bring to a simmer, then remove from heat.
Heat broiler and broiler pan. Place meat on pan and broil 2 to 3 minutes on each side for medium-rare (the longer a skirt steak is cooked, the tougher it gets). Remove from broiler and set on a carving board. Let rest 5 minutes.
Meanwhile, return saucepan to medium-high heat and bring just to a simmer. Add rosemary and season with salt and pepper to taste. Strain sauce.
To serve: Cut steak into thin slices across the grain. Spoon sauce over meat and serve.
Recommended ingredients: Dark amber maple syrup is preferred (available online at www.bairdfarm.com).