Preheat oven to 350 and butter a 9'' springform pan. Prepare the rhubarb that will be placed on top of the cake by slicing it in half lengthwise and then cutting each stalk at different lengths to fit into the pan (see video above for more). I use another cake pan or trace the cake pan onto parchment paper and line up the rhubarb to do this.
Place a fine mesh strainer over a medium bowl. Add flours, baking powder, and salt into strainer and lightly shake the strainer over the bowl to sift the flour direclty into the medium bowl. Set aside.
In a large bowl, whisk eggs, egg yolk, and sugar. Vigorously hand whisk for about a minute, until noticeably light in color.
Add in melted butter, skyr/yogurt, vanilla extract, and lemon zest into the bowl with the eggs and whisk to combine.
Add in flour mixture and whisk gently to fully combine. Pour batter into pan.
Option 1: Place rhubarb on top before baking- do this if don't mind if the rhubarb sinks just a bit and doesn't show as much. Place cake in fridge for 15 minutes to let firm up a bit before placing on rhubarb. Add rhubarb stalks on top and sprinkle with turbinado sugar. Place bake in oven and bake for 40 minutes, until top is golden brown.
Option 2: Bake cake for just a bit to firm up and then place rhubarb on top. You have to move quickly on this, but it will allow the rhubarb to stay on top and be more noticeable (like cover photo).Bake for 7 minutes to firm up just slightly. During this time, cut the rhubarb. First, slice the rhubarb in half lengthwise. Then cut into lengths that will evenly fit in the pan. I will either use another cake pan or trace the cake pan onto parchment paper and line up the rhubarb to make sure it will fit.Remove cake and as quickly as possible, add rhubarb stalks on top and sprinkle with turbinado sugar. Place bake in oven and bake an additional 38-45 minutes, until top is golden brown.
Let cake cool completely on a cooling rack before removing from pan.