Place the yeast, water, milk and 1 tablespoon of sugar in a large bowl and set aside in a warm place for 10 minutes or until bubbles appear on the surface. Add the butter, flour, eggs and remaining sugar to the yeast mixture and use a butter knife to mix until a sticky dough forms. Turn out onto a lightly floured surface and knead until smooth. Place the dough in a lightly oiled bowl, cover with a clean, damp cloth and set aside in a warm place for 45 minutes or until doubled in size.Knead the dough on a lightly floured surface for 5 minutes or until smooth and elastic. Roll 1 tablespoonful of dough into balls at a time and place on a baking tray lined with non-stick baking paper and set aside for 30 minutes or until risen.Place the oil and a sugar thermometer in a large, deep saucepan over medium heat until temperature reaches 180°C (350°F). Cook the doughnuts, in batches, for 1 minute each side or until golden. Drain on absorbent paper. Place the custard in a piping bag. Carefully pierce doughnuts with a knife and fill with the custard. Serve immediately. Makes 30.