Add flour, baking powder, sugar and salt to processor bowl and pulse to mix. Add diced butter and pulse until the mixture forms small crumb clusters. Add hazelnuts and pulse 4-5 times to mix; avoid over processing. Place in a shallow dish. If you prefer larger clusters, run crumbs between your fingers for desired texture. Refrigerate until ready to use. Wipe food processor clean.
If using frozen rhubarb, do not thaw. Add rhubarb and sugar to food processor bowl and pulse until coarsely chopped. Transfer to a medium sauce pan set on low heat. Allow rhubarb to release some juice, add grapefruit juice and increase heat. Once juices boil, reduce to medium heat and cook, stirring occasionally, until juices have evaporated and the rhubarb is tender, approx. 10-12 min. Stir in zest and set aside to cool.
In a small mixing bowl, whisk egg and sugar until dissolved. Add ricotta and cornstarch and mix well.
Preheat oven to 350 degrees. Line bottom of springform pan with parchment paper. Place 3 packed cups of streusel into pan and lightly press, creating a crust. Top with ricotta mixture and bake 15 min. Remove from oven and let cool 5 min.
Gently spread rhubarb and grapefruit compote on top of ricotta and sprinkle with remaining crumbs, but do not pack down. Bake 25 min, then rotate pan and bake 20 min more, until crumbs are golden brown and bubbling compote appears thick.
Let cool 20-30 min. Release the spring and remove cake from pan, transferring to a serving plate or cake stand. Dust with powdered sugar, if desired.