For Softening the Rhubarb
2 pounds rhubarb, washed, trimmed and cut in 1-inch pieces
¾ cup zucchero di canna, or white sugar
For the Pan
3 tablespoons soft butter
2 tablespoons zucchero di canna, or white sugar
1¾ cups all-purpose flour
2 teaspoons baking powder
⅛ teaspoon salt
8 tablespoons soft butter
1 cup sugar
½ teaspoon vanilla
1 cup milk
10-inch pie pan;
The night before baking the cake (or at least 8 hours), put all the rhubarb in a large bowl and toss with the sugar to coat all the pieces. Let the fruit steep as long as possible, then drain through a sieve or colander; save the exuded rhubarb juice. The pieces should be soft and reduced to 4 cups in volume. (If they haven't had time to wilt, put the fruit in a skillet with a 1/2 cup or so of the juice; bring to a simmer and cook for 5 minutes, or until the pieces are soft, have released more juice, but still have their shape. Drain and cool, again saving the juice.)
Arrange a rack in the lower third of the oven and preheat to 375 degrees-you can bake this on a stone, if you have one.
Smear the butter to coat the insides of the pan with a thick layer of butter on the bottom. Sprinkle the 2 tablespoon of sugar over the bottom (not the sides). Spread the drained rhubarb pieces in an even layer in the prepared pan; press gently just to flatten the layer.
Sift or stir together the flour, baking powder and salt.
With the mixer on low speed, cream the butter and sugar together in a big bowl; when blended, beat on high speed for a couple of minutes, until smooth and light. Scrape down the sides of the bowl and beat in the eggs, one at a time, mixing well at moderate speed, and then mix in the vanilla. Beat on high speed for 2 minutes or so to lighten and smooth the batter.
Scrape down the sides and, at low speed; mix in the dry ingredients alternating with the milk. Add a third of each at a time and beat for a few seconds before the next addition. When everything's been incorporated, scrape the sides (and bottom) of the bowl and beat on high speed for about a minute until light and completely smooth. Pour the batter over the layer of rhubarb and spread it evenly to fill the pan and to level the top.
Set the pan in the oven and bake for 25 or 30 minutes until the top of the cake has set and is starting to brown. If using a glass pan, lift it carefully and check to see that the fruit is cooking and bubbling on the bottom. (If the fruit is not cooking actively but the top is browning, keep baking and lay a piece of foil loosely over the cake before it gets too dark. If the juices are bubbling so much that they're about to spill over the sides, place a sheet of foil under the dish.)
Bake for 45 to 50 minutes, or until both the cake and fruit are done. The cake should be set so that a tester or toothpick inserted into the middle comes out clean; the top should be golden brown. The fruit juices should be bubbling up around the sides of the cake and appear syrupy.
Remove the cake and cool on a rack for about an hour. The pan should be comfortable to hold but still slightly warm when you invert it.
Meanwhile, during the time the cake is baking or cooling, put all the reserved rhubarb juice in a saucepan, bring it to a boil and cook uncovered until it has thickened into a ruby-colored thick syrup.
To unmold the cake, run the blade of a sharp knife around the sides of the pan. Cover the cake top with a round serving plate and, holding tight, flip the pan and plate upside down. The cake and rhubarb layers may drop neatly onto the plate. It they're reluctant, give the pan a quick shake to release the fruit and pry one side of the cake from the pan to let air in and encourage it to drop.
Serve the cake still slightly warm or at room temperature. Slice it into wedges and drizzle over a bit of the rhubarb syrup. If you want, top with unsweetened whipped cream or flank with a scoop of vanilla ice cream, or just enjoy it plain, with a cup of good coffee.