In a large bowl, combine the olive oil, garlic, balsamic vinegar, basil, salt, and pepper. Stir and refrigerate until cold, 1-2 hours.
Slice bread slightly on the diagonal into ½-inch thick slices. Preheat a griddle or large skillet and melt half the butter. "Toast" half the bread on both sides in the butter. Repeat with the other half of the stick of butter and the rest of the bread.
Serve the bread crostini on a large platter with the bowl of the tomato mixture alongside. Spoon tomatoes and the juice over slices of the bread. Serve immediately.
The bread can be toasted up to 2 hours in advance and the tomato mixture can be made and refrigerated no more than 1-2 days in advance.