Preheat the oven to 220ºC/425ºF/gas 7. Place a pizza stone or a large baking tray in the oven to heat up.
Heat the milk in a small pan over a low heat until lukewarm, then place 50ml into a jug with the yeast and sugar. Mix well, then set aside for a few minutes until starting to bubble.
Meanwhile, sieve the flour, xanthan gum and salt into a large bowl, then make a well in the middle. In a separate bowl, combine the egg, 3 tablespoons of olive oil and the remaining milk, then pour it into the well, along with the yeast mixture. Gradually bring the mixture together with a fork until it forms a smooth dough.
In a small cup, combine the bicarbonate of soda and vinegar, then quickly knead it into the mixture. Place the dough into a lightly oiled bowl, cover with a damp tea towel, then leave to prove in a warm place for around 1 hour, or until doubled in size.
Meanwhile, make the topping. Pick the basil leaves and set aside, then finely chop the stalks and the garlic. Heat a splash of olive oil in a large frying pan over a medium heat, then add the basil stalks and garlic. Cook for a few minutes, or until golden. Pour in the tinned tomatoes, break them up with the back of a wooden spoon, then cook for 5 to 10 minutes, or until thickened. Transfer to a blender along with half the reserved basil leaves, blitz until smooth, then season to taste.
Once doubled in size, divide the dough into four equal-sized pieces on a flour-dusted surface. Roll out until roughly 30cm in diameter and 2mm thick. Place the pizza bases onto the preheated pizza stone or baking tray (you’ll need to do this in batches), then spread over the tomato sauce, leaving a rough 2cm gap around the edge. Tear over the mozzarella, then pop in the hot oven for 10 to 12 minutes or until golden and crisp. Scatter the reserved basil leaves on top, then serve.