Sift the flour, salt and baking powder into a large mixing bowl. Rub in butter until mixture resembles breadcrumbs.
Add the water and mix until the dough can be gathered together. If necessary, add more water, 1 teaspoon at a time. Cover the dough with a damp cloth and let rest for 15 minutes.
Divide the dough into 12 equal pieces and roll into balls. On a lightly floured surface roll and stretch each round into a circle 35cm across.
Cook in a large, dry frying pan over medium-high heat until brown spots begin to appear on the surface, then turn and brown the other side. Keep tortillas covered with a clean, dry cloth until ready to serve.