Adapted from Not Quite Nigella's "Spectacular Upside Down Blood Orange Cake."
Preheat the oven to 325 F. Lightly grease an 8-inch cake pan with butter and then line it with a large (14- to 15-inch diameter) circle of baking parchment. The parchment will line the whole pan--not simply cover the bottom.
Using the sharpest knife you own (or a mandoline), thinly slice 2-4 blood oranges. You want slices no more than 1/8 inch thick.
In a medium non-reactive saucepan, combine 1/2 cup water and 200 g (1 cup) sugar over medium low heat, stirring occasionally until the sugar is dissolved. Once the sugar is dissolved, raise the heat to medium-high and add the blood orange slices. When the mixture comes to a boil, lower the heat to medium and simmer the blood orange slices for 10-15 minutes, until the pith is translucent.
Use a slotted spoon to remove the candied blood orange slices to a large plate or tray to cool slightly. Bring the syrup in the saucepan back to a boil and boil for 3-5 minutes, until further reduced and thickened. Transfer to a bowl and allow to cool to room temperature.
Meanwhile, begin lining the parchment-lined pan with the candied blood orange slices, allowing them to slighty overlap. Line both the bottom of the pan and the sides, bending the slices as necessary where the bottom and sides of the pan meet. If the blood orange slices are not adhering to the sides of the pan, use a little of the reduced blood orange syrup to help them stick. Set the pan aside while you make the sponge.
Make the sponge: Using an electric hand mixer or a stand mixer with the paddle attachment, cream together the butter, sugar, and grated blood orange zest until light and fluffy. Add the eggs, one at a time, mixing well after each and scraping down the bowl as necessary. Mix in the optional vanilla.
In a medium bowl, whisk together the almond flour, all-purpose flour, baking powder, and salt. Set aside.
Using a wooden spoon or silicone spatula, mix the yogurt into the batter. Now, fold in the dry ingredients; make sure they are well incorporated but avoid overmixing.
Spread the batter in the prepared pan, taking care not to dislodge the orange slices. Smooth the top, then transfer to the oven and bake 45-50 minutes, until the top is golden and a wooden skewer inserted in the center comes out clean.
Allow the cake to cool in the pan for 10 minutes, and then turn out onto a plate or serving platter. Gently remove the parchment. After the cake has cooled somewhat (20-30 minutes more), brush on some of the reserved blood orange syrup. Allow the cake to cool completely before slicing.