Whisk egg yolks and sugar in a bowl until it turns soft peak. Add in mascarpone cheese and continue whisking on medium until smooth.
Transsfer stout beer in a shallow dish. Place six quartered ladyfingers in the stout mixture until soaked, turning as needed. Transfer hald of soaked ladyfingers to a glass or ramekin. Spoon ¼ cup of mascarpone mixture, then repeat another layer until the glass or ramekin is filled completely with mascarpone mixture as the top layer. Cover with a plastic wrap and refrigerate overnight.
Just before serving, place cocoa powder in a fine mesh sieve and shake a light dusting over the mascarpone mixture