Combine yeast, sugar and warm beer in a bowl and set aside in a warm place until foamy (20 minutes). Transfer to an electric mixer fitted with a dough hook, add flour, cream, egg and a large pinch of salt and knead until a soft dough forms. Transfer to a lightly oiled bowl, turn to coat, cover and set aside in a warm place until doubled in size (1 hour).
Roll out dough on a lightly floured surface to 1cm thick, then cut out rounds with a 5cm-diameter pastry cutter. Place on a lightly floured tray, leaving space between each, then re-roll scraps and repeat. Set aside until slightly risen (10-20 minutes).
Meanwhile, for pastry cream, whisk yolks and sugar until pale, add flour and whisk to combine. Bring milk, cream and vanilla to the simmer in a saucepan over high heat, add to yolk mixture, whisk to combine, then return to pan and whisk continuously over medium-high heat until bubbling and thick (3-5 minutes). Cover closely with plastic wrap and cool to room temperature, then transfer to a piping bag fitted with a 4mm nozzle.
Heat oil in a large saucepan or deep-fryer to 170C, then deep-fry doughnuts in batches, turning occasionally, until puffed and golden (3-5 minutes; be careful, hot oil will spit). Drain on absorbent paper, insert nozzle of piping bag into one side of each doughnut and fill with pastry cream. Dust with caster sugar and serve warm or at room temperature.