Start by making the pastry. Place the flours into the bowl of the food processor along with a pinch of sea salt. Add the diced butter and pulse a few times until the butter is the size of peas. Add the egg and the cold water and set the processor running just until the dough forms a ball. As soon as it comes together, turn off the processor and tip the dough out onto a floured surface. Gently press the dough into a disc, wrap in cling film and place in the fridge for 20 minutes.
Preheat the oven to 200°C (400°F).
In a small bowl, mix together the soft butter and the caster sugar. Add a good pinch of sea salt. Smear this mixture over the base of your mini skillets/ramekins/baking dish.
Pit the cherries and arrange them over the butter and sugar mixture. I use 8 cherries per mini skillet.
Roll out the pastry to about 3mm (1/8") thick. Cut out rounds of pastry that are slightly larger than the tops of your mini skillets/ramekins. Lay the pastry rounds over the fruit and tuck in the edges using the end of a spoon.
Place the tartes tatin on a baking tray and bake in the preheated oven for 50 minutes or until the pastry is very golden brown and crisp and the caramel is syrupy.
Remove from the oven, carefully turn out and serve with plenty of ice cream.