In a small bowl, combine the warm water and yeast. Let it sit for about 5-10 minutes until it is foamy.

In a large mixing bowl, add the buttermilk, egg, and melted butter. Whisk until combined.

In another bowl, whisk together the flour and sugar.

Make a well in the center of the dry ingredients and pour in the wet mixture along with the activated yeast. Mix until a dough forms.

Transfer the dough onto a floured surface and knead for about 5-7 minutes until it is smooth and elastic.

Place the dough in a greased bowl, cover it with a cloth, and let it rise in a warm place until it doubles in size, about 1-2 hours.

Roll the dough out on a floured surface to about 1-2 cm thick, and use a donut cutter to cut out shapes.

Place the shaped donuts on baking sheets, cover with cloths, and allow them to rise again for 30 minutes.

Heat oil in a deep fryer or large pot to 180°C (350°F). Fry donuts until golden brown, about 2-3 minutes on each side.

Remove from oil and let them drain on paper towels. Once cooled, fill each donut with custard using a piping bag.