Custard: scald all but 2 T of milk in a saucepan, add sugar, stir until dissolved. Beat egg yolks with that 2 T milk and flour. Gradually pour 1/2 sweetened hot milk into the yolk mixture, whisking constantly. Return mixture to saucepan, cook over medium heat stirring continuously. Once boiling begains, continue stirring for 2 minutes, then remove from heat. Strain custard into a bowl, whisk in butter, liquor and vanilla. Cover with wax paper or plastic to prevent skin fomration. Refrigerate until cold. Cut cake into 3 horizontal layers. Place bottom layer into 9springform pan. Brush cake layer with espresso. Top the layer with 1/3 of custard. Add second cake layer, brush with espresso, top with 1/2 of remaining custard. Add final cake layer, top with remaining custard, sprinkle cocoa. Chill for 1 hour.