Preheat the oven to 325º.
Melt butter in a large cast iron pan over medium heat. Be careful not to burn the butter. Add brown sugar and stir until mixed. Remove from heat. Add vanilla, honey and salt, then stir to combine.
Add the fruit to caramel mixture, cut-side down-fit as much as you can inside. (NOTE: Place the kumquats in a single layer and place a tin foil-lined cookie sheet on the rack below the pan to catch any overflow.) Set aside.
Put softened butter and sugar in a large bowl and beat until fluffy. Add eggs one at a time, beating on high. Add vanilla.
In a separate bowl, sift together dry ingredients. Add to wet ingredients and mix on low speed until combined.
Carefully spoon or pour batter over kumquats, taking caution not to disturb the fruit. Carefully smooth the batter.
Place cake in oven on the middle rack. Bake for about 1 hour, checking after 45 minutes. Test the cake with a toothpick, making sure it is cooked in the middle. Take a butter knife and loosen edges along the pan. Put cake on a wire rack and let it cool for about 30 minutes.
Put a large platter face down over cast iron pan and flip.