Heat the oven to 200 C / 390 F. Line a sheet pan with parchment paper.

Unroll the puff pastry on a lightly floured surface and cut it into 18 equal squares, roughly 6 x 6 cm each.

Score a smaller square about 1 cm inside the edge of each piece using a sharp knife - press firmly but do not cut all the way through.

Dock the inner square of each piece 3 to 4 times with a fork to prevent it from rising.

Place the squares on the prepared sheet pan. Brush the scored border of each square lightly with beaten egg, avoiding the inner square.

Bake for 18 to 22 minutes until the borders are deep golden brown and the inner square is cooked through but flat. Transfer to a wire rack and cool for 10 minutes.

If the inner squares have puffed up, press them down gently with the back of a teaspoon to create a well for the caramel.

Pour the granulated sugar into a heavy-bottomed saucepan over medium heat. Stir constantly with a wooden spoon until the sugar fully melts and turns a clear amber color, about 6 to 8 minutes.

Once fully melted and amber, stop stirring. Add the cold butter cubes all at once and whisk vigorously until fully melted and combined, about 1 minute.

Remove the pan from the heat. Slowly pour in the heavy cream while whisking - the mixture will bubble up sharply, so keep whisking until smooth.

Add the fine sea salt and whisk once more. Let the caramel cool for 4 to 5 minutes until it thickens slightly but is still pourable.

Spoon about 1 teaspoon of warm caramel sauce into the well of each pastry shell.

Scatter a pinch of flaky sea salt over each filled pastry, if using.

Dust the edges lightly with powdered sugar just before serving. Serve warm or at room temperature.