A sumptuous, moist spice cake made with calamondin/calamansi fruit and fresh-grated ginger.
First, prepare calamondin topping. Pour melted butter into an 8" diameter × 2" deep aluminum cake pan and swirl it around to get bottom pan edges coated. Evenly sprinkle brown sugar over the butter. Arrange sliced calamondin pieces on top of the brown sugar. If you use calamondin end pieces, be sure to arrange them so that fruit side is facing bottom of pan for attractive final appearance. (See pictures in article.)
Butter sides of cake pan now or wait until you remove from fridge in later step. Next, place pan in the refrigerator for 30+ minutes so that the toppings set. This helps ensure they don't move as batter is poured and leveled.
Preheat oven to 350°F (177°C).
Prepare the cake batter. In a mixing bowl, whisk the dry ingredients together (flour, baking powder, baking soda, cinnamon, and salt). Set aside.
Using an electric mixer with whisk attachment, beat the butter on high speed for about a minute until it's smooth and creamy. Add the brown sugar and beat on high speed until creamed together, about 1 minute. (Use a silicone/rubber spatula as-needed to remove ingredients from side and bottom of mixer.)
Next, beat in egg whites on high speed, then yogurt, grated ginger, and vanilla extract. Use the spatula again to scrape ingredients off sides and bottom of mixing bowl and mix again until fully incorporated.
Pour the dry ingredients into the wet ingredients. Turn the mixer to low speed and as the mixer runs, slowly pour in the milk. Continue mixing on low speed until all of the ingredients are combined. Use the spatula again to make sure no unmixed ingredients are left on side or bottom of mixing bow.
Remove toppings/cake pan from fridge. If sides of pan are not yet buttered, do it now. You want a generous coating to ensure the sides of the cake don't stick to the pan.
Slowly pour cake batter over chilled topping - a spatula helps to remove it from the bowl and evenly spread it over the surface of the calamondins.
Bake for 20 minutes, then put a tent of aluminum foil over top of cake before baking for another ~27 minutes. This prevents the top of the cake from burning as the insides finish cooking.
You'll know the cake is done when: 1) the center has risen, 2) it's no longer jiggly, and 3) a toothpick doesn't have any liquid dough on it. Note there will be some sticky crumb, since this is a very moist cake.
Cool cake on a baking rack until it's only slightly warm or room temperature before turning it out. Or you can wait longer - the cake pictured in this article was made late at night and turned out the following morning.
Before turning it out on to cake plate or serving dish, run a butter knife along the edges of the pan to loosen it. Place upside down on cake plate and pop the back of the pan firmly with your hand. Cake should dislodge with all or almost all of the toppings perfectly intact. You can do repairs to topping as shown in article.
For best presentation: 1) slice cake with a serrated knife (a dull knife won't cleanly cut through the calamondins), and 2) wait until the cake has had plenty of time to cool.
Calamondin and ginger upside down spice cake https://www.tyrantfarms.com/recipe-calamondin-ginger-upside-down-spice-cake/