Preheat oven to 350 degrees F. Line a 10-inch tart pan (with removable bottom) with parchment paper. Spray with non-stick spray.
With the skin still on, slice citrus into 1/2-inch thick slices. Use a paring knife to carefully remove the rind from the citrus slices, cutting off the outer ring and keeping the citrus slices intact. Slice the kumquats into thin slices, remove the seeds, but keep the rind on.
Mix 1/4 cup of sugar with 1/4 cup melted butter and spread half of the mixture on the bottom of the prepared pan. Next, line the bottom of the pan with the citrus fruits slices, arranging them decoratively. Once arranged, pour the remaining sugar and butter mixture over the citrus. Set aside.
In a large mixing bowl, beat the 1/3 cup granulated sugar, 1/3 cup brown sugar and 1/2 cup butter together until light and fluffy. Add the eggs, egg white, citrus juice, citrus zest and vanilla extract and mix until combined.
In a separate bowl, combine the flour, cinnamon, baking powder, baking soda, and salt, whisking to combine. Slowly alternate folding in the flour mixture and the yogurt into the wet ingredients until everything is combined. Mixture will be thick.
Pour the batter over the prepared citrus slices, spreading it evenly to the edges.
Bake for 35 minutes until golden brown and a skewer comes out clean when poked into the center of the cake. Allow to cool completely before inverting onto a serving plate.
Serve and enjoy!
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