The claim that chocolate milk aids recovery after exercise is supported by scientific research, including a 2018 systematic review and meta-analysis published in European Journal of Clinical Nutrition by Amiri et al. (PMID: 29921963).The meta-analysis reviewed 12 controlled trials comparing chocolate milk (CM) to water, placebo, and commercial sports drinks on recovery markers such as time to exhaustion, heart rate, serum lactate, and creatine kinase (CK). 
Three key findings I found were, the time to exhaustion (TTE) significantly improved after CM consumption compared to placebo and nutrient-matched beverages. Another was serum lactate levels were significantly lower with CM versus placebo, suggesting better metabolic recovery.
Lastly, no major differences were found for heart rate, perceived exertion, or CK compared to other sports drinks.
Overall, the study concluded that chocolate milk provides equal or better recovery benefits than many commercial recovery drinks, but noted the need for larger, high-quality trials.Other supporting evidence includes research from University of Texas at Austin (Karp et al., 2006) and Journal of Strength and Conditioning Research (2010) showing similar findings on endurance and muscle recovery improvements with CM.
Sources: 
- Amiri M, Ghiasvand R, Kaviani M, Forbes SC, Salehi-Abargouei A. Chocolate milk for recovery from exercise: a systematic review and meta-analysis of controlled clinClin Nutr. 2018;72(11):1617–1627. DOI: 10.1038/s41430-018-0187-x
 
 
- Karp JR et al. Chocolate milk as a post-exercise recovery aid. Int J Sport Nutr Exerc Metab. 2006;16(1):78–91.