Hi B.,
I'm a personal trainer, as well as a fitness competitor, here are my suggestions...
Squats: You will need to tire out that behind muscle! Choose a heavy weight and do at least 3 sets of 12. On the last set, and last repetition exceed the 12, and go till you are tired.
Deadlifts: These are a great lower back move, as well as glute move. 3 sets of 10-12 here with a heavy weight as well. Using a lighter weight will cause you to cut the muscle, but tiring out the muscle will gradually give you a build.
Short Distance Sprints: Ever notice how sprinters have great behinds? I never knew til I realized its because of their sprints! Short distance sprints of about 100 m will get that butt booming
Lunges: Lots of lunges as well both with weights and without. The weights with these do not necessarily have to be heavy. Do stationary, walking, side lunges. These will help to target both your glute and legs.
Lastly, all of these factors work hand in hand with diet! 5-6 meals a day, especially since you say you have a thin frame and a hard time gaining weight, you could even get away with eating 6-7 meals a day.
Hope this helps! You can message me if you have any questions. Wish you the best of luck on getting that booty back!