En Passant...putting King in check?

If taking a pawn En Passant gives a discovered check to an opponents king...can you do it? I can't figure out why I cant do it.

Sure. As long as it doesn't put your own King in check.

Here's a chess "problem" for you...

White played 1. Bg2+ and said "Mate next move".

Black chuckled and replied "Next move? How about THIS move?" and played 1. ... d5 discovered check.

White captured the Pawn en-passent, and Black objected "Wait, wait... you can't do that! You're in check!"

White replied, "No, YOU'RE in check. Your Pawn never reached the d5 square. It was captured en-passent while crossing the d6 square. You can't check me... you never blocked my original check."

The two players argued for a while, then called over the arbiter.

Who was awarded the point?

Black.  the pawn made it to d5. capturing en passant is not a legal move with white's king in check.

d5 shouldn’t be legal for black. But I believe it is.

Black should be awarded the point. The one giving the check is his Bishop, not his pawn. cxd does not put the white king out of check, while the d5 move does that to the black king.

In an en passant, the concept is that the pawn never gets to d5 as you capture it as if it had only moved one square.

Why should d5 be illegal?

Black seems to be the winner.

Because conceptually it doesn't block the check...

I understand what you say and you have a point. We're talking about a matter of mili-tempos here (I hope my made up term makes a bit of sense). The moment the white pawn is ready to cut down its black counterpart, it gets informed that the king is mated, thus the attack never happens. It is one of the paradoxes of the pawns "defying time" when in 2nd or 7th rank and moving twice as fast as usual.

Or to see this in another way: we know that the motto of tactics is "Checks, Captures and Threats" in that order. So the check has always the most powerful effect. Thus, whenever two effects happen simultaneously (in this case, a check and a capture), the check gets priority and happens first.

And FWIW, I put the above position in the Chess Analysis, and it agrees with me, d5 is perfectly legal and an instant win for Black. 

He 'cuts down the black counterpart' as it reaches the square. The king isn't yet under attack.

The moment the pawn is moving, the king comes under attack.

Oh yeah sorry. In that case, using your logic, the moment the black pawn is ready to move he gets informed the king is mated.

If it never reached the d5 square, then capturing en passant is illegal

The pawn reaches the d5 square.  Capturing en passant is (generally speaking) a matter of choice.  It's not automatic.  In the above case, it's not a matter of choice.

We get how it works from a legal chess point of view...

^he does not get informed about a mate, he is informed about a check. He is also informed that the only way to get out of check, is for him to move two squares forward to d5, which is a legal move.

Which game? Need to see it if we're going to figure out why.