How do you atone for forgetting a good friend's birthday?

I swear, I thought it was tomorrow, but it was today.
Damn.
This is a kindred-spirit (and coworker) who always has my back and has taught me so much about friendship and loyalty.

I fucked up. He didn’t say anything at all to anyone at work today. He texted me afterwards saying he was feeling low.

Right then, I realized how much I suck as a friend.

I’m prepared to stay up all night baking a cake and (lamely) decorating it… but how do you really make up for the hurt?