Sieve the flour together with the baking powder and sugar.
Work the cold butter into the mixture until it looks like bread crumbs.
Add the egg.
Mix to make a dough. Cover in plastic wrap and let it rest for an hour in the refrigerator.
TipUse a food processor to mix the ingredients together. Pulse a few times for flour, baking powder, sugar, and butter to create a mixture that resembles bread crumbs. Add the egg and pulse a few more times until you obtain a dough.
In the meantime wash and dry the plums, cut them in halves and remove the pits.
Preheat the oven to 350F/180C.
Use an 8x8 inch pan. (You do not need to butter the pan, as the dough has enough fat in it).
Spread the dough on the bottom of the pan and place the plums on top of the dough in dense rows. Sprinkle them with the crumbs and bake for 40-45 mins. (the dough has to be baked, plums should still sweat)
Let the pan cool, then cut the bars in squares and serve. Sprinkle powder sugar on top if you prefer.
Notes
Store the cake in an airtight container and a dry environment for up to 3 days.