Don't play 7. cxd4, just castle and if they take on c4 play 8. Qb3. It's scary to hand off the second pawn but it passes the "Bobby Fisher played it" test. On top of that, hang onto that dark squared bishop if you can. A big reason you give up the b pawn is to get that dark squared bishop some nice activity, even preventing castling at times by landing on a3.
Evan's Gambit Accepted Line 5.cxd4 Bb4+

I looked at this in detail with some chess engines once, and I don't remember my conclusions, but I agree with 7. O-O.
However I disagree with the follow-up to 7. ... dxc3.
In that case, I think its 8. Bxf7 Kxf7 9. Qd5+; this is with the intention of recapturing a bishop and then the pawn.
However if the king retreats back to e8, I think you can throw in a zwischenzug of Qh5+ to entice out g7 (to g6) making the rook vulnerable.
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I think my analysis may have been that 6. O-O is better than the 6. d4.
As I seem to remember, black has adequate defences against attacks like 6. Qb3 and 6. d4. The defending moves might have been bringing out the black queen and bringing out the knight from g8.
So while each attack could be adequately defended, the quiet little castling move sort of forced black to choose his defence before white chose his attack. The result was that black finds himself with an inappropriate defence for white's attack.
I am sorry that I cannot be more specific. I looked at 5. ... Ba5 and 5. ... Be7 lines etc and its now all a blur in my mind. I have confused which opening demands early white castling.
I looked at it all with Stockfish and Houdini and other chess engines over a period of time, so my conclusions were probably much better than one would think, given my grade.
Although I love Evan's gambit for white, it does seem that with best play, black wins. Here is one line from a game, but I have lots of these and the conclusion from putting them all together was that white will lose out against best play.
ed | e4 | e5 | Nf3 | Nc6 | Bc4 | Bc5 | b4 | Bxb4 | c3 | Bc5 | Qb3 | Qe7 | d3 | Nb4 | Ng5 | h6 | Bxf7+ | Kf8 | Nh7+ | Rxh7 | Bxg8 | Nxd3+ | Kd2 | Nxc1 | Kxc1 | Rh8 | Nd2 | Rxg8 |
ed | e4 | e5 | Nf3 | Nc6 | Bc4 | Bc5 | b4 | Bxb4 | c3 | Bc5 | Qb3 | Qe7 | d3 | Nb4 | Ng5 | h6 | Bxf7+ | Kf8 | Nh7+ | Rxh7 | Bxg8 | Nxd3+ | Kd2 | Nxc1 | Kxc1 | Rh8 | Nd2 | Rxg8 |
I usually follow up with Bd2, what do you guys suggest?