In a medium sized bowl, combine rhubarb, ¼ cup sugar, and lemon zest. Let sit while you make the dough.
In a large bowl, mix together flour, baking powder, and salt. Using your fingers or a pastry cutter, cut in the butter until it forms small pea-sized lumps. Add in remaining sugar, rhubarb and hazelnuts and mix until the fruit is evenly scattered through.
In a small bowl, whisk together vanilla, heavy cream, and 1 egg. Pour into flour mixture and use your hands to knead and form dough until it comes together, adding additional cream if mixture is too dry.
Divide dough in half and transfer to a floured surface. Form each half into a 6 inch disk and cut each into 6 triangular scones
Arrange all 12 scones on a baking dish. In a small bowl, whisk egg yolk with a couple drops of water and brush tops of scones. Bake for 20-25 minutes, until tops are browned. Let cool slightly before eating.