For chocolate custard, whisk egg yolks and 80gm sugar in a bowl until thick and pale, sift over flour and stir to combine. Combine milk, remaining sugar and vanilla bean and seeds in a saucepan and bring to the boil over medium heat. Pour a third of the milk mixture over egg yolk mixture, whisk to combine, then return to remaining milk mixture in pan. Stir over low-medium heat until thick and starting to bubble (8-10 minutes). Remove from heat and stir in chocolate until smooth and combined. Cool completely, then refrigerate until required.
Combine yeast and warm milk in a small bowl, stir until yeast dissolves and set aside in a warm place until foamy (3-5 minutes). Beat butter and sugar in an electric mixer until pale and creamy, transfer to a bowl and set aside. Combine flour, eggs, 2 tsp salt and yeast mixture in a clean bowl of an electric mixer fitted with a dough hook and blend on medium speed until smooth and elastic (8-10 minutes). Add butter mixture, a little at a time, allowing each addition to be incorporated before the next. Beat until smooth and glossy (6-8 minutes). Cover with a clean tea towel and stand in a warm place until double in size (1½-2 hours). Turn and press dough with fingertips to knock back, cover and refrigerate for 2 hours.
Preheat oven to 190C. Place dough on a lightly greased and floured baking tray and press out to a 25cm-diameter circle, cover loosely with a clean tea towel and set aside in a warm place until double in size (45-60 minutes). Brush top with egg wash, scatter with flaked almonds and bake in the centre of oven until golden brown (35-40 minutes). Cool on a wire rack, then halve horizontally, spread cut-side of one half with chocolate custard, sandwich with remaining half, dust with icing sugar and serve.