Rob Kardashian surfaced for a rare appearance at Khloe Kardashian 40th birthday bash this week, RadarOnline.com can reveal. The seldom-seen member of the mega-famous family was spotted in an Instagram video Khloe posted on Thursday, sitting between his grandmother, Mary Jo Campbell, and sister’s best friend, Malika Haqq. Rob, 37, looked chipper as the small group cheered for Khloe on her milestone birthday. Surrounded by loved ones and lavender-colored decorations, the reality star blew out the candles on her matching ornate cake. “Deeply Grateful For Every Moment,” Khloe captioned her post, w…