Preheat oven to 350°. Arrange croissant bottoms in a greased 13x9-in. baking dish. Sprinkle with chocolate chips; replace croissant tops.
In a large bowl, whisk eggs, egg yolks, cream, sugar, vanilla and, if desired, coffee liqueur; pour over croissants. Let stand until croissants are softened, 15 minutes.
Bake, covered, 30 minutes. Bake, uncovered, until puffed, golden and a knife inserted near the center comes out clean, 30-40 minutes longer. Cool in pan on a wire rack 10 minutes before cutting. Serve warm.