To make the pastry, combine the flour, baking powder and 1 teaspoon of sea salt in a large bowl. Finely chop or coarsely grate the butter, then rub into the dry mix until it resembles fine breadcrumbs. Mix in just enough cold water to bring it together, then wrap in clingfilm and pop in the fridge for 1 hour.
Meanwhile, finely chop and add the sirloin with 1 tablespoon of oil to a large non-stick frying pan over a medium-high heat. Fry for 1 to 2 minutes, then remove to a plate.
Peel and finely chop the garlic and onion. Deseed and finely chop the pepper and chilli, then add to the frying pan over a medium-low heat. Stir in the paprika and cumin, and fry for 10 minutes, or until softened. Meanwhile, crush the olives with the flat side of your knife, pull out and discard the stones, then finely chop the flesh.
Add the purée to the pan and crumble in the stock cube. Fry for a further minute, then add the olives and 100ml of boiling water. Bring to the boil, season lightly with sea salt and freshly ground black pepper.
Cook for 8 to 10 minutes over a medium heat, or until the liquid has almost completely evaporated. Return the beef and any resting juices to the pan, stir well, then leave to cool completely.
Preheat the oven to 190°C/375°F/gas 5. Divide the pastry into 14 equal pieces, then roll each portion into 14cm circles and the thickness of a pound coin – use a biscuit cutter or saucer for an accurate round.
Spoon 2 tablespoons of the filling onto the middle of each pastry round, brush the edges with a little beaten egg, then fold them over the filling to make a semi-circle. Press the edges together with a fork to seal, then place onto a large, lightly greased baking tray.
Brush the empanadas with the beaten egg and bake in the hot oven for 25 to 30 minutes, or until golden and crisp. Leave to rest for a few minutes, then serve on a platter.