Position rack in center of oven. Preheat oven to 350°F/180°C. Coat the inside of a 9-inch (23 cm) round springform pan with nonstick spray and dust with the extra flour, tapping out any excess. Set aside.

Whisk the 3/4 cup (109 g) plus 2 tablespoons of the flour together with the baking powder and salt in a small bowl to aerate and combine; set aside.

Beat the 1 1/4 cups (248 g) of sugar, eggs and egg yolk with an electric mixer in a large bowl for a few minutes or until very thick and fluffy and pale yellow. Beat in vanilla and lemon zest, then beat in the melted butter and sour cream very gently.

Fold the dry mixture into the batter and scrape into prepared pan, spreading it so that it is even and level. Lay the raw rhubarb, side by side, across the top of the batter, trimming to fit the pan as you go. Each piece of rhubarb should reach from one edge to the other and you can use a couple of pieces to assemble each “stripe” of rhubarb. Sprinkle top with remaining 1 tablespoon of sugar.

Bake for about 40 to 50 minutes or until cake is golden brown and a toothpick inserted in the center tests clean. Cool on rack for 10 minutes, then run a small icing spatula between the cake and the pan (pressing out towards the pan) to loosen the cake. Remove the sides of the springform pan and cool cake a bit more. This cake is best barely warm or at room temperature. We like to serve this cake the day it is made. It can be stored overnight at room temperature loosely covered with foil.