Mix the cold mashed potatoes with the flour, 4 tablespoons of powdered sugar and the baking powder to form a stiff dough, leave to cool for half an hour. Remove the skin from the apples, remove the core and cut into 1 cm thick slices. Grease a cake springform pan (approx. 20 cm ø).
Divide the dough in half, roll out one half and use it to cover the bottom of the tart springform pan. Place the apple slices on the dough, sprinkle with cinnamon and 1 tablespoon of sugar and drizzle with honey. Place the rest of the dough on top, make 3 to 4 deep incisions and glaze with a little water and 2 tablespoons of powdered sugar.
Bake in the oven heated to about 190 °C for about 30 to 40 minutes until golden brown.
Enjoy the baking!