Trim and discard tough tops and root ends from lemon grass. Remove and discard tough outer layers. With the flat side of a knife, crush tender inner stalks; cut crushed stalks into 2-inch pieces. In a 1 1/2- to 2-quart pan over high heat, combine lemon grass pieces, butter, ginger, and garlic. When butter is melted, turn heat to low and stir often until flavors are blended, 15 to 20 minutes. With a slotted spoon, lift out and discard lemon grass. Use butter hot. If making up to 1 day ahead, let cool, cover, and chill; reheat to serve.