Mix the dry ingredients: In a large mixing bowl, whisk together the flour, caster sugar, baking powder, bicarbonate of soda, and salt. The mixture should look pale and uniform, with no lumps of baking powder. Set aside.

Combine the wet ingredients: In a separate jug or small bowl, whisk together the buttermilk, egg, melted butter, and vanilla extract. The mixture will be a pale yellow colour and smell sweetly of vanilla. Pour this into the dry ingredients and stir with a wooden spoon until just combined. The batter will be thick, slightly sticky, and look a bit like a soft scone dough. Do not overmix, or the doughnuts will be tough.

Heat the oil: Pour the vegetable oil into a large frying pan or skillet, about 1cm deep. Heat over a medium flame for two to three minutes. To test if it is ready, drop a tiny pinch of flour into the oil. If it sizzles and bubbles immediately, the temperature is correct. If it burns straight away, turn the heat down slightly.

Shape and fry the doughnuts: Using lightly oiled hands, scoop out portions of the dough (about the size of a golf ball) and gently shape them into a patty shape, about 1.5cm thick. Carefully place them into the hot oil, leaving space between each one. Fry for two to three minutes per side. The underside should turn a deep golden brown, and you will hear a gentle sizzling sound. When you flip them, the cooked side should feel firm to the touch. Work in batches of four to avoid overcrowding the pan.

Drain and cool: Using a slotted spoon, transfer the cooked doughnuts to a plate lined with kitchen paper. They will be crisp on the outside and soft inside. Let them cool for five minutes while you make the glaze, or they will melt the chocolate later.

Make the chocolate glaze: Place the dark chocolate pieces, double cream, and golden syrup in a small heatproof bowl. Microwave in 20-second bursts, stirring each time, until the chocolate is fully melted and the mixture is smooth and glossy. It should look like a thick, dark sauce and smell intensely of chocolate. Alternatively, set the bowl over a pan of simmering water.

Assemble the doughnuts: Using a small, sharp knife, cut a slit into the side of each cooled doughnut, creating a pocket. Spoon or pipe the vanilla custard into a piping bag (or use a strong sandwich bag with the corner snipped off) and fill each doughnut until it feels heavy. Dip the top of each filled doughnut into the warm chocolate glaze, letting the excess drip off. Place them on a wire rack for the glaze to set for a couple of minutes before serving.