Take 2 tablespoons of the milk in a bowl and add 1 tsp of the sugar and the yeast. Mix well and set aside.
In the bowl of a stand-mixer with the dough attachment, add the flour, salt, sugar and the remaining milk. Mix till the flour is moistened and let it sit for 5-10 minutes.
After 5-10 minutes, your yeast mixture should be light and fluffy. Add this to the flour and start mixing at a low speed, slowly increasing to high. Knead for 2-3 minutes, then add the condensed milk. Knead for another 2-3 minutes. In case your dough is too dry, add 1-2 tbsp of water.
Lastly, with the mixer on low, gently add the soft butter. Once poured, increase the speed to high and knead till a very smooth and elastic dough forms, it should not stick to the sides of the bowl and will be very smooth.
Roll the dough into a smooth ball and place it in a greased bowl. Cover and let it proof till it is doubled in size. Time can range from 45 minutes up to 2 hours depending on the temperature of your kitchen. Do not over proof your dough.
Once doubled in size, drop the dough to a clean and floured surface, roll the dough out using a rolling pin to approximately an 2 cm thickness. Take a round cookie cutter (I used a 3 inch cutter) and cut out as many circles as you can. You can re-roll the scraps once and make more doughnuts, I do not recommend doing this more than once.
Place the cut out doughnuts on a floured tray and cover with a damp cloth. Let them rise for another 30-45 minutes or until almost doubled in size.
Fry the doughnuts
In a large heavy bottomed pot, fill enough oil so that it comes 3/4 way up. Let the oil heat up to 175 C and fry the doughnuts at the most 3 at a time. Let them cook for 2-3 minutes per side or until light golden brown.
Coat the fried hot doughnuts in caster sugar for the classic look.
Make the custard according to package instructions and let it cool completely
Fill the doughnuts:
Use a knife or skewer to poke a hole through the donut, only pierce halfway through the donut and be gentle not to break it.
Fill the custard into a piping bag fitted with a narrow piping tip. Squeeze the filling into the warm doughnut till it is full.