Mix the flour and salt in a bowl, then add the cold butter. Use your fingers or a pastry cutter to work the butter into the flour until it resembles coarse crumbs.
Gradually pour in the ice-cold water, stirring gently until the dough begins to come together. Knead it briefly on a floured surface until smooth, then wrap in plastic wrap and chill for 15 minutes.
While the dough chills, heat olive oil in a skillet over medium heat. Add the chopped onion and cook until fragrant and translucent, about 3 minutes.
Add the ground beef and cook until browned and cooked through, about 5-7 minutes. Stir in cumin and smoked paprika, then season with salt and pepper to taste. Let the mixture cool slightly.
Roll out the chilled dough on a floured surface to about 3mm thickness. Use the round cutter to cut out 12 circles, keeping the dough chilled for easier handling.
Place a teaspoon of the cooled filling in the center of each dough circle. Fold the dough over to form a semi-circle, then press and crimp the edges with a fork to seal tightly.
Brush the top of each empanada with the beaten egg, giving them a shiny, golden appearance after baking.
Arrange the empanadas on a parchment-lined baking sheet and bake in a preheated oven at 200°C (390°F) for 20-25 minutes, until golden brown and crisp.
Remove from the oven and let the empanadas rest for 5 minutes to settle. Serve warm with your favorite dipping sauce or a squeeze of lime.
