Butter bottom and sides of springform pan, then line bottom with parchment paper.
Sprinkle gelatin over 1/4C. water in a small bowl and let soften, about 1 minute
Beat yolks with an electric mixer until very pale. With mixer at low speed, add sherry, 1/4C. sugar, and remaining 2TBS. water. Cook mixture in a small heavy sauce pan over medium-low heat, whisking constantly, until very thick, 3 to 5 minutes.
Whisk in gelatin mixture until gelatin has dissolved. Quick chill by putting sauce pan in an ice bath. Cool mixture to room temperature, stirring occasionally, then remove pan from ice bath.
Beat cream, vanilla, and remaining 2TBS. sugar in a large bowl with cleaned beaters until mixture just holds soft peaks.
Fold in sherry mixture gently but thoroughly, then fold in cake cubes.
Transfer to springform pan and smooth top. Chill, covered with plastic wrap until set ( at least 4 hours. )
Run a thin knife around cake and remove side of pan.