Preheat oven to 350 degrees F. Grease an 8-inch spring form cake pan and set aside.

To make citrus topping, heat the butter over medium-low heat in a small skillet. Add citrus slices, sugar, and sea salt. Cook 7 minutes, stirring occasionally, until a sticky caramel sauce has formed. Finish with a splash of Prosecco if using, and stir to incorporate. Keep over lowest heat while you prepare batter.

Meanwhile, de-stem, seed, and rough chop kumquats reserved for the cake. Puree in a food processor, combining with the milk/yogurt mixture. Set aside.

In the bowl of a stand mixer fitted with a paddle attachment, beat butter on low until light and fluffy. Add sugar, and continue beating until well-combined and pale. Add eggs one at a time, scraping down sides as you go. Next add sea salt, baking powder and soda, and then half of the flour. Add half of the kumquat/dairy mixture, then the remaining flour, and finally, the remaining liquid.

Spoon into prepared cake pan. Carefully arrange citrus slices on top of cake (steer clear of edges if you can, since citrus along the edges tends to burn). There will be several tablespoons of liquid left in the skillet––keep it over low heat while the cake cooks.

Set cake on a rimmed baking sheet and bake 50 - 60 minutes, or until a wooden toothpick inserted in the center comes out with just a few crumbs attached.

Cool cake 10 minutes, remove spring form ring, and drizzle with reserved citrus/caramel liquid. Cool, slice, and serve.