What's the best way to cool down after a grueling workout? With a Double Double of course!

Hours before her mother's extravagant 60th birthday party, Kylie Jenner took her juicy booty to the gym and then hit In 'N' Out in Sherman Oaks, Calif., for some meat.

The celebutante was ushered around town by her main squeeze, Tyga, who drove up to the foodie-approved fast food joint in his $320 Ferrari.

Earlier in the day, Kylie hit the gym in a sheer leotard -- that girl loves to show off that round rump of hers!

On Friday night, the two switched up their off-duty ensembles for some fancy duds. The twosome were quite the pair at Kris Jenner's "Great Gatsby" themed birthday bash, which reportedly cost $2 million. We don't always love Kylie's style -- but we thought she looked like a diamond goddess at the jazzy soiree.