Peanut Upside-Down Cake with Caramelized Topping

Pan preparation

1. Preheat the oven to 175 °C (350 °F) with the rack in the center.
2. Line the bottom of a rectangular metal pan about 22 x 11 x 7 cm with parchment paper cut to size. Use a one-piece pan, not removable-bottom. Grease the paper and the pan walls well with butter.

Topping

3. Chop the peanuts into halves or coarse pieces. You can do this with a knife or by putting them in a bag and hitting them a few times with a rolling pin. Do not turn them into powder.
4. In a small saucepan, combine the butter, brown sugar, and water over medium-low heat. Stir until the sugar dissolves. Let it boil with small bubbles for 1-2 minutes, until thickened and glossy. Do not darken it further: it will finish caramelizing in the oven.
5. Remove from heat, add the peanuts, and mix to coat them well. Immediately spread them on the bottom of the pan in an even layer and press down slightly with the back of a spoon.

   

Batter

6. In a bowl, beat the softened butter with the peanut butter and brown sugar for 2-3 minutes, until you have a lighter, fluffier cream.
7. Add the eggs one at a time, beating well after each one. Incorporate the vanilla.
8. Sift together the flour, baking powder, and salt. Add them to the cream in three parts, alternating with the milk in two parts; start and end with the flour. Mix on low speed or with a spatula only until no dry flour is visible. Do not overbeat.
9. Spoon the batter over the topping, spreading it across the entire pan without disturbing the peanuts. Gently smooth the surface.

   

Baking and flipping

10. Bake for 55-60 minutes. If after 35 minutes you see the surface browning too much, cover it loosely with aluminum foil.
11. Check doneness around 55 minutes. Insert a skewer into the center to a depth of about 3-4 cm, without reaching the bottom; it should come out clean or with dry crumbs. The surface should be golden and spring back when pressed. Do not mistake the sticky syrup on the bottom or edges for raw batter. If the skewer comes out with wet batter, bake 5 minutes more and test again.

    

12. Let the cake rest in the pan on a rack for 10-15 minutes. It should still be warm so the caramel does not stick.
13. Run a thin knife along the sides to loosen it. Place a rectangular serving dish upside down over the pan and flip everything in one firm motion, using cloths to avoid burning yourself. Wait 1 minute, lift the pan, and carefully remove the paper. If any peanut pieces stuck, place them back with a spoon while still warm.

    

14. Let cool for at least 1 hour before slicing, so the topping settles and the crumb firms up. Slice with a serrated knife into 2-2.5 cm slices.