Step 1Place the butter in a medium saucepan and bring to a simmer over medium-high heat. Scrape the vanilla bean and add both the bean and the seeds to the butter. Continue simmering until the butter browns to a deep russet color, about 20 minutes.
Step 2Remove from the heat and divide equally into two small bowls, discarding the bean. Chill the browned butter thoroughly, until it is once again solid. This may be done a day ahead. Use half the brown butter in the cake; reserve the rest for sauce.
Step 3Heat the oven to 350 degrees. Place parchment in the bottom of a 9-by-5-inch loaf pan and spray the sides with nonstick spray. Sift together the flour, baking soda and salt.
Step 4In the bowl of an electric mixer, using a paddle attachment, cream together one-half of the chilled brown butter and 1 1/2 cups of the sugar. Continue mixing until the butter is light in color and very fluffy in texture, about 3 minutes on medium speed.
Step 5Add the eggs one at a time, allowing each to be fully incorporated into the batter and scraping down the sides of the bowl.
Step 6On the slowest speed, add half of the sifted flour mixture. Mix just until combined. Add the creme fraiche and mix until combined. Add the remaining flour mixture and mix just until fully incorporated. Fold in the zest.
Step 7Pour the batter into the pan. Bake for 30 minutes. Rotate and continue baking until a tester comes out with small, defined crumbs sticking to it, about an additional 50 minutes. The top will be somewhat crunchy. Remove the cake from the oven and using a toothpick, make 15 to 20 punctures in the top. Cool for 15 minutes.
Step 8Combine 3 tablespoons of the lemon juice and the remaining one-fourth cup of sugar in a small bowl. Spoon the syrup over the top of the cake and allow it to be absorbed into the cake. The cake may be served as soon as 25 minutes after baking, or the next day.
Step 9With a sharp paring knife, peel the blood oranges by slicing the tops and bottoms off and then slicing off the peel from top to bottom. Cut segments out of the orange over a small bowl to catch all the juices. Peel the Clementines by hand and slice into pinwheels. Slice the kumquats into small, evenly shaped circles.
Step 10Combine the blood orange juice with the honey and the remaining lemon juice.
Step 11Melt the reserved browned butter.
Step 12To serve, place a slice of cake on each plate and spoon brown butter over the slice and onto the plate. Place three or four segments of blood orange on the other side of the cake and pour a spoonful of juice-honey sauce over the fruit. Arrange the Clementine and kumquat slices on the plate with the blood oranges. Sprinkle the decorative sugar over the fruit. If desired, place a tiny scoop of ice cream on the fruit salad.