In a small bowl, add all marinade ingredients and whisk until combined. Pour over flank steak in a large (gallon size) ziptop bag. Refrigerate at least 4 hours, but preferably overnight (I actually prefer to marinate mine for 24 hours).
When ready to grill, beat or stir all butter ingredients in a small bowl until smooth. Set aside at room temperature until ready to serve.
Prepare grill for direct heat on high. Sear flank steak about 2-4 minutes on each side, depending on the thickness and the desired doneness. Two minutes for rare to medium rare, 4 minutes for medium well..
Remove from grill and allow to rest, tented with tin foil, for 10 minutes. Slice along the width, against the grain, into thin slices. Place pats or spoonfuls of the chili butter over the warm slices. Serve immediately with extra chili butter on the side.
Notes
Flank steak can be marinated up to 24 hours. Butter also goes well with grilled corn on the cob or oven browned potatoes.
Keyword chili butter, chili flank steak, Gluten Free