Line round 2 1/2-inch collars with strips of acetate and place them in the freezer.
In a large bowl, whisk the yolks, sugar, brandy, malt powder, milk, and vanilla bean together. Over a hot water bath cook whisking constantly until very thick, like a sabayon. Remove from the heat and take out the vanilla bean. Whisk in the chocolate to melt it, then let this cool almost completely. Fold in the whipped cream until completely combined.
Pour semifreddo mixture into collars filling 1/8-inch from the top and freeze overnight. To serve it, take it out of the collar and sprinkle it lightly with cocoa powder and powdered sugar then place on dessert plates then peel off the acetate. Let sit for 10 minutes before serving to temper it a bit.