This rustic rhubarb version of the classic Irish apple cake falls somewhere between a scone and a cake, although it actually looks like a pie. The sweet-tart fruit filling is sandwiched between layers of tender scone dough, which seal in and absorb the rhubarb juices. The cake is best served warm with a lashing of whipped cream.
If you're using hothouse red rhubarb, the sugar quantity given here for the filling will be about right. If your rhubarb is a greenish-red variety, increase the sugar to 1 to 1-1/4 cups unless you like your desserts (and rhubarb) on the tart side.
Position a rack in the middle of the oven and preheat the oven to 375 degrees F. Butter a 10-inch deep-dish pie plate and set aside.
Combine the rhubarb, sugar, and orange zest in a bowl and stir to blend. Set aside at room temperature while you make the scone dough.
Place the flour, baking powder, sugar and salt in a large bowl and whisk to blend. Add the butter and cut it in with a pastry cutter or with your fingers, breaking up the butter until the mixture resembles coarse meal.
In another small bowl, whisk together the orange zest, buttermilk and egg yolk. Set the egg white aside for the garnish.
Add the liquid ingredients to the bowl with the dry mixture and stir until just combined. Gather the dough in your hands and squeeze it lightly to form a cohesive mass. Don't overwork the dough or your scone cake will be tough.
Lightly flour a work surface and divide the dough in half. Pat one half into a flat disk, then roll it out into a 12-inch circle. Transfer the dough to the prepared pan. Add the rhubarb mixture, including any juices. Pat the remaining piece of dough into a flat disk, and roll it into a 12-inch circle. Transfer the dough to the pie plate, arranging it atop the rhubarb filling. With a knife or scissors, cut the edges of the top and bottom pastry rounds so they are even with edge of the pie plate. Press the edges together to seal the dough. Alternately, you can wet the edge of the bottom pastry and then press the top layer against it to adhere. Avoid creating a thick, bulky crust.
Cut 3-4 slits in the top dough to allow steam to escape during baking. Lightly whisk the reserved egg white with a teaspoon of cold water, and brush some of the egg wash over the top crust. Sprinkle the wet crust with the 2 tablespoons of sugar.
Bake until the cake is golden and the rhubarb is tender when pierced with the tip of a knife, 40 to 50 minutes. Remove the cake from the oven and let it cool on a wire rack for at least 15 minutes before serving. Cut the cake into wedges and serve warm.