Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper to prevent sticking.

In a medium bowl, combine the dried currants, melted butter, dark brown sugar, mixed spice, and lemon zest, stirring until the mixture is uniform and the fruit is well coated.

Unroll the puff pastry sheet on a lightly floured surface and use a round cutter (about 4 inches in diameter) to cut out 8 circles, re-rolling scraps if necessary.

Place a generous spoonful of the currant mixture into the center of each pastry circle, leaving a small border around the edge.

Dampen the edges of the pastry with a little water, then gather the edges up over the filling and pinch them together tightly to seal the bundle.

Turn the cakes over so the seam is on the bottom, then gently flatten each one with a rolling pin until the currants just start to show through the dough.

Arranging the cakes on the prepared baking sheet, brush the tops with the beaten egg white and sprinkle generously with the granulated sugar.

Use a sharp knife to make three small slashes across the top of each cake to allow steam to escape during baking.

Bake in the preheated oven for 15 to 20 minutes, or until the pastry is puffed, golden brown, and crispy.

Remove from the oven and transfer to a wire rack to cool slightly before serving warm or at room temperature.