For candied lemon rind, remove rind from lemons with a vegetable peeler, remove any white pith with a small sharp knife, then cut peelings into 5mm dice. Blanch in boiling water for 30 seconds, then repeat twice more. Drain and set aside. Juice lemons and combine juice in a small saucepan with 165gm caster sugar and 165ml water. Bring to the simmer over medium heat, stirring to dissolve sugar, then add rind and simmer until tender and translucent (15-20 minutes). Strain though a fine sieve (reserve syrup for drizzling, if desired), then combine in an airtight container with remaining sugar and shake to coat rind in sugar, ensuring rind doesn’t clump together. Set aside.
Preheat oven to 220C. Combine flour, sugar, baking powder, lemon rind and 1 tsp salt in a bowl and make a well in the centre. Add milk, 2 tbsp olive oil and 1 tbsp liqueur, stir to combine, then turn onto a lightly floured work surface and knead lightly to form a smooth ball. Cover with a tea towel and rest for 20 minutes.
Divide dough into walnut-sized pieces and roll out each one as thinly as possible on a lightly floured work surface. Place on baking trays lined with baking paper, prick with a fork, then brush with remaining oil. Scatter with candied lemon rind and bake in batches until golden (12-15 minutes). Drizzle with remaining liqueur and reserved syrup, if desired, and serve warm or at room temperature. Tortas de aceite will keep, stored in an airtight container, for about a week.