Plum Upside-Down Cake

Step 1
Preheat the oven to 350 degrees F (175 degrees C). Drop butter into an 8-inch square and let melt in the preheating oven.

Step 2
Remove the pan from the oven and tilt to coat the bottom evenly with butter. Sprinkle brown sugar and almond flour on top. Arrange plum slices over the mixture.

Step 3
Beat softened butter and sugar together until light and creamy. Mix in egg and vanilla extract until fluffy.

Step 4
Sift flour, baking powder, salt, and cinnamon together in a separate bowl. Add to the butter mixture alternately with the milk, folding just until dry spots disappear. Pour batter evenly over the plums; tap pan onto the counter to expel excess air bubbles.

Step 5
Bake in the preheated oven until a toothpick inserted into the center comes out clean, 40 to 45 minutes.

Step 6
Cover the pan with a serving plate and immediately invert cake onto the plate. Let cool until no longer hot, about 15 minutes. Cut and serve warm.