In a pot, combine almond milk, sugar, salt, and oil. Heat until the sugar is dissolved.

Let the mixture cool slightly, then add the egg (or egg replacer), vanilla extract, and lemon juice.

Transfer the mixture to a stand mixer and add the sourdough starter. Gradually add the flours and knead for 10 minutes, until a smooth dough forms.

Place the dough in an oiled bowl, cover with plastic wrap and a towel, and let it rise at room temperature for eight to 10 hours. Then, transfer it to the fridge overnight.

Remove the dough from the fridge and let it come to room temperature (about one hour).

Roll out the dough and cut into round shapes using a cup. Reroll the scraps and repeat until all the dough is used.

Cover the donuts with a towel and let them rise at room temperature for two to four hours, until they look slightly puffy.

While the donuts are rising, make the custard filling. In a pot, combine almond milk and cornstarch. Cook over low heat, stirring constantly, until the mixture thickens (about five minutes).

Remove the custard from the heat and stir in granulated sugar, vanilla extract, and lemon juice. Let it cool to room temperature.

Heat the oil in a deep frying pan to 350 degrees Fahrenheit (175 degrees Celsius). Fry the donuts until golden brown, then remove from the pan and immediately sprinkle with powdered sugar.

Let the donuts cool slightly before filling with the custard using a piping tool.