Preheat oven to 325 degrees. Butter the bottom and sides of a large round cake pan, or use a 9-inch springform pan, lined with parchment paper and buttered.
Mix the rhubarb, cornstarch and 1/2 cup granulated sugar together in a medium bowl.
Mix the brown sugar and 1/2 stick butter in a pan over medium heat until smooth and bubbling, about 2 minutes. Sift together the cake flour, baking powder and salt.
Whip 2 sticks butter in a mixer for 2 minutes (we suggest using a paddle attachment). With your fingers, blend the remaining 1 cup sugar with lemon zest until the mixture is a consistent color. With the mixer set to medium-high, cream the sugar and lemon zest mixture together with the butter until it is light and fluffy, about 4 minutes, stopping to scrape down the bowl halfway through. Add the vanilla and mix well.
Add the eggs, one at a time, mixing well each time. Stir in the yogurt, then the lemon juice. Set the mixer to low speed and add the flour mixture, 1/4 cup at a time, until well combined. Scrape the mixer bowl in between additions.
Pour the brown-sugar mixture into the cake pan, then spoon in the rhubarb and its juices. Gently pour the batter over the rhubarb and smooth out the top.
Bake for 1 hour and 15 minutes, or until the top of the cake is firm and a toothpick comes out clean.
Cool the pan on a wire rack for 15 minutes. Run a knife around the cake, place a plate on top of the pan and turn it upside-down. Release the cake from the pan while still warm, so it doesn’t stick.
Delicious garnished and served with sweetened whipped cream or vanilla ice cream.
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