- Preheat the oven to 375 degrees F.
- Grease a 9 inch springform cake pan with butter, and sprinkle it with a little flour to lightly coat the bottom and edges.
- Mix together the flour, baking powder, salt, and lemon zest and place them in a mound on a clean surface. Cut the frozen butter first into thin slabs, then long rectangles, and finally into very small cubes, about 5 to 10 millimeters. It is easiest to do this if you coat the butter and knife with some of the flour; this prevents the knife from sticking too much. Once the butter has been cut, place in the freezer for 5–10 minutes to re-firm. Spread the chilled butter cubes around the periphery of the flour mound.
- Make a well in the center of the flour. Add the sugar, egg, milk, and vanilla extract to the well. Scramble these together using a fork, then slowly incorporate the surrounding flour, using the fork to stir it in.
- When the mixture becomes too thick for the fork, use a large knife to cut in the rest of the flour and butter. Continue cutting the dough together, remembering to scrape under and turn over the dough during this process. Do this for a couple of minutes, until the dough is in the form of large, crumbly lumps.
- Wash and flour your hands. Briefly knead the dough until it is no longer sticking strongly to the board. If the dough is wet, sprinkle on a little extra flour. Do not overknead. You still want to see the little pieces of butter in the dough; this will produce a flaky crust.
- Form the dough into a ball, wrap it in plastic, and place it in the fridge for 15 minutes. After its brief chill, flour your work surface and roll the dough out so that it is about 1½ inches larger than the cake pan. Transfer the dough to the pan and press on it lightly, so that it makes contact with the bottom and sides. Ideally, the border of the dough should come an inch up the sides of the cake pan. If the dough is not high enough, use a floured fork to gently pull it up to an inch in height. Using the tines of a fork, poke the bottom and sides of the dough a number of times, so that it can breathe while baking. Place the dough in the fridge to rest for 30 minutes, and prepare the plum filling.
- In a medium bowl, mix together the bread crumbs, ground almonds, lemon zest, sugar, and cinnamon. Sprinkle half of this (about 1 /2 cup) on the bottom of the pastry crust.
- To prepare the plums, cut them in half lengthwise—but leave the back edge of each fruit intact, so that it folds open like a book. Pry out the pits.
- Stand the plums upright in the pastry bottom, making concentric circles, working from the outside of the pastry inward until you reach the center. If there are any large gaps, fill them in with any remaining extra plum halves.
- Sprinkle the second half of the almond mixture over the top of the plums. Slice the cold butter into thin slivers, and dot them over the top of the tart. Sprinkle with the slivered almonds.
- Bake the tart for 40 to 55 minutes, until the pastry is a dark golden brown and is pulling away from the sides of the pan. By this time, the plums should have nestled down into the pastry and released their juices. Serve with lightly whipped cream, or warmed with a scoop of vanilla ice cream.