It was a series of panels starting with modern day Bucky remembering when pre-serum Steve brought him birthday cake that was then shoved into Bucky's face, next was him regarding a beaming big Steve bearing cake with suspicion. Steve just continues to offer him that piece, then the final panel was Sam bringing & smushing cake into Bucky's face.
It was possibly on either tumblr or twitter, although searching both hasn't turned anything up. Anyone recognise it?
Edit 27/10/16: Found, on Twitter!