Mix the flour and salt in a bowl. Add the water and butter, knead quickly to form a soft, smooth dough. Flatten the dough, wrap in cling film, chill for approx. 30 mins.
To laminate the dough
On a lightly floured surface, roll the dough out into a rectangle (approx. 10 x 20 cm). Spread the butter over two thirds of the dough, leaving a border of approx. 1 cm all around. Fold the exposed dough over the top, place the folded dough over the remaining dough. Press the edges down firmly. Press out any air pockets using a rolling pin. Wrap in cling film and chill for approx. 30 mins.
On a lightly floured surface, roll the dough into a rectangle (approx. 10 x 20 cm), then fold up one third of the dough and fold the top third down over the top. Repeat this step three more times. Wrap the dough in cling film and chill for approx. 30 mins.
To prepare the caramel
Spread the sugar and 1 tbsp of water in each of the prepared trays. Caramelize for approx. 7 mins. in the centre of an oven preheated to 220 °C until the sugar turns a light brown colour.
Spread the butter over the caramel, place the plums (slightly overlapping) on top. Divide the dough in two. On a lightly floured surface, roll each half into a circle (approx. 20 cm in diameter), prick firmly with a fork, lay loosely over the plums, press down gently around the edges.
Approx. 20 mins. in the centre of an oven preheated to 220°C. Remove from the oven and leave to stand for approx. 2 mins., carefully tip the tarts out onto plates, remove the baking paper. Drizzle with the juice, serve the tarts warm or cold.