Make the lemon syrup first. Put 500 ml (17 fl oz/2 cups) of water and the sugar in a saucepan over medium heat and bring to the boil. Add the lemon juice, lower the heat and continue to simmer for 10 minutes. Remove from the heat and pour the syrup into a deep bowl. Set aside.
In a separate saucepan, mix 500 ml (17 fl oz/2 cups) of water with the butter, salt and sugar, and bring to the boil over medium heat. Add the flour and continue to boil for 3 minutes, stirring constantly, to make a dough. Remove from the heat and stir to help it cool. Set aside.
Put the dough in a mixing bowl and stir in the eggs, one at a time, until combined. Knead for 3 minutes. Add the semolina and cornflour, and knead for a further 5 minutes to make a soft moist dough, adding 1 tablespoon of water if it seems dry. Cover the bowl with a damp cloth and set aside to rest for 15 minutes.
Push the dough into a piping bag with a star nozzle. Squeeze out a long line of dough and cut into 6 cm (2½ in) chunks. Pour the vegetable oil into a deep heavy-based saucepan. Add the tulumbas and place the pan over low heat. Once the oil starts bubbling, simmer for 5 minutes or until the donuts turn golden brown. Remove the tulumbas with a slotted spoon and transfer into the bowl with the lemon syrup. Leave to soak for 5 minutes, then remove from the syrup and serve.