Yes, the Parc des Buttes Chaumont is open at night, but only for access to Rosa Bonheur! This mysterious listed guinguette, takes its name from a famous 19th-century lesbian painter. Although it retains its traditional appearance, it serves as a restaurant/ bar in the daytime and at nightfall becomes a gathering place for hip Parisians. It is especially popular at apero time and you shouldn’t be deterred by the queue to get in.
On the other hand, it is strictly forbidden to walk in the Parc des Buttes-Chaumont at night! The proprietors of the restaurant/bar rely on their clients to respect these rules. Cf the charter of Rosa Bonheur, respected to the letter …