SAY THIS
"Thank you again for the offer. I'm really excited about joining the team and I believe I can make a strong contribution from day one. I've done some research on the market rate for this level of role and, given my [X years of experience / specific achievement], I was hoping we could explore something closer to [BND X]. Is there any flexibility on the base salary?"
Short, professional, confident. "Is there any flexibility" is softer than "can you match X" and tends to open a more receptive response. Your experience is the anchor, not your current salary.