Preheat the oven to 350 degrees F. Grease 6 mini cocotte pans and set aside.
In a saucepan over medium heat, add ¼ cup of butter. Cook until foam appears and a nutty scent develops. Approximately 5 minutes.
Add 100 g of light brown sugar, cognac, and lemon juice; stir until sugar melts, about 3 minutes. Evenly divide the mixture into the prepared mini pans.
Zest 3 blood oranges. Then, remove the outer layer of one of the blood oranges. Slice the blood orange crosswise into 1/4-inch-thick wheels; discard any seeds as well as the outer layer.
Place one blood orange wheel on top of the cognac brown sugar mixture. Set aside.
In a bowl sift together the salt, baking soda, baking powder, buckwheat flour, and cake flour.
In a separate bowl beat together the remaining unsalted butter, light brown sugar, and granulated sugar until light and fluffy, approximately 3 minutes.
Beat in the eggs, one at a time. Next, beat in the vanilla extract, blood orange zest, and sour cream. Fold in the dry ingredients.
Evenly divide the cake batter into the prepared mini cake pans. Use an offset spatula to gently spread the batter into a smooth and flat layer.
Bake the mini cakes for 20 to 35 minutes or until inserted toothpick comes out clean.
Let the cakes cool for 10 minutes before inverting onto plates. Enjoy!