Blackberry Jam and Custard Donuts

First, place all the ingredients for the donuts, except the butter, into the bowl of a stand mixer fitted with the dough hook. Work on a low speed for about 4 minutes, or until well combined and elastic.With the mixer still running, add the butter piece by piece, until it is all worked in and incorporated. There should be no visible pieces. This will take about 5-8 minutes.Scrape down the sides of the bowl, cover with plastic, and set aside in a warm place to rise until doubled in size.While the dough is rising, make the filling. Place the cream, milk and vanilla into a small saucepan, and place over a medium heat.In a small bowl, whisk together the egg yolks, caster sugar, and cornflour. When the milk begins to bubble around the edges, remove it from the heat, and slowly whisk it into the egg mixture.Pour the mixture back into the pan, and place over a medium heat, whisking constantly. Cook until the mixture boils and becomes very thick - about 3 minutes. Once the mixture is the consistency of soft butter, scrape it out into a bowl, cover and set aside to cool completely.When the dough has risen, punch it down, and scrape it out onto a well-floured surface, pat the dough into a rectangle, about ½ inch (just over 1cm) thick, and cut out 9 donuts using a 2.5inch (5cm) round cutter. Place the dougnuts on a lined baking sheet, cover with plastic again, and set aside to rise for another 45 minutes, or until puffy.When the donuts are finished their second rise, place about 2 inches of oil into a high-sided pan (or use a deep fryer), heat the oil over a low flame, until it reaches 170C. Alternatively, place a small piece of dough in the oil, and when it bubbles and rises to the surface, the oil should be the right temperature.Fry the donuts a few at a time (don't crowd the pan) for about 1 minute each side or until they are golden brown and cooked through.Drain the donuts on paper towels, and when they are all fried, toss them in the extra caster sugar.To fill the donuts, ripple the cooled pastry cream with the blackberry jam, and then place the mixture into a piping bag fitted with a ¼ inch (1/2 cm) nozzle. Press the piping bag into the side of each donut, and squeeze until you can feel the weight of the donut increase slightly. Continue until all the donuts are filled, and then cram one in your face immediately!