Preheat oven to 400 degrees. Butter an 8-inch-round, 2-inch-deep cake pan and line bottom with parchment; butter parchment. Place 1 cup yogurt in a fine-mesh sieve set over a bowl and let drain in the refrigerator.

Meanwhile, toss together rhubarb, 1/2 cup granulated sugar, 1 tablespoon butter, and 1/4 teaspoon salt on a rimmed baking sheet. Roast, stirring once, just until tender, about 15 minutes. Remove, reserving rhubarb and juice. Reduce oven temperature to 325 degrees.

In a food processor, finely grind pistachios. Add flour, baking powder, and remaining 3/4 teaspoon salt; pulse to combine.

In a medium bowl, beat together remaining 1 stick butter and 1 cup granulated sugar with a mixer on medium-high speed until light and fluffy. Beat in eggs, one at a time, scraping after each addition. Add the orange-blossom water, vanilla or extract and beat for a moment to combine.

Reduce speed to low and beat in flour mixture in two additions, flour first, alternating with one cup of the yogurt. Transfer batter to pan and smooth top.

Using a slotted spoon, remove about half the rhubarb from the rhubarb syrup mixture and place over the top in a single layer. (Refrigerate remaining rhubarb in syrup until ready to serve.)

Sprinkle cake with granulated sugar and bake until a tester inserted in center comes out clean, about 1 hour, 15 minutes. (If browning too quickly, tent with foil.) Let cool in pan on a wire rack 15 minutes. Run a small sharp knife around edges. Invert onto a cutting board, then immediately flip onto rack; let cool completely.

To serve, stir confectioners’ sugar into drained yogurt. Add the remaining yogurt and some of the syrup to the yogurt and stir. Reserve the rest of the syrup. When ready to serve, top the cake slices with the yogurt mixture and drizzle with the remaining syrup.