1. Prepare the Pan and Oven:

Preheat the oven to 350°F (175°C).

Spray a 9-inch round cake pan with baking spray with flour.

Line the bottom of the pan with parchment paper. Do not spray the parchment again.

2. Prepare the Rhubarb:

Cut the rhubarb into 2-inch diagonal pieces.

Cut each piece in half widthwise so that each piece has a pink side and a green-white side. This creates a visually appealing pattern.

Place the rhubarb in a large bowl.

Sprinkle with 2 tablespoons of granulated sugar and the cornstarch.

Toss to combine.

Place the rhubarb in the prepared pan, pink side down, trimming the pieces to fit snugly in the pan.

3. Make the Caramel:

In a medium stainless-steel skillet, melt ¼ cup of the butter over medium heat.

Stir in the brown sugar, honey, and ¼ teaspoon of the salt.

Bring the mixture to a simmer. Cook until the sugar is melted and fragrant, about 3 minutes.

Slowly pour in the orange juice, whisking until combined and smooth.

Pour the caramel evenly over the rhubarb in the pan.

4. Make the Cake Batter:

In the bowl of a stand mixer (or using a hand mixer and a large bowl), beat the remaining ½ cup of softened butter, the remaining 1 cup of granulated sugar, the orange zest, and the vanilla extract at medium speed until creamy, 2 to 3 minutes. Stopping occasionally to scrape the sides of the bowl.

Add the eggs, one at a time, beating well after each addition.

In a medium bowl, whisk together the all-purpose flour, baking powder, ground ginger, baking soda, and the remaining ½ teaspoon of salt.

Gradually add the flour mixture to the butter mixture alternately with the sour cream. Beginning and ending with the flour mixture. Beat just until combined after each addition. Be careful not to overmix.

5. Pour Batter over Rhubarb: Carefully dollop the batter onto the rhubarb in the pan.

Gently spread it with an offset spatula or the back of a spoon, making sure not to press too hard or move the rhubarb pieces.

6. Bake:

Bake for 30 minutes.

Place a piece of foil on top of the cake to prevent excess browning.

Bake until a wooden pick inserted into the center of the cake comes out clean, 15 to 20 minutes more.

7. Cool and Invert:

Remove the foil.

Let the cake cool in the pan on a wire rack for 10 minutes.

Run a thin knife or offset spatula around the sides of the cake to loosen it from the pan.

Carefully invert the cake onto a serving plate.

Remove the parchment paper.

Let the cake cool completely.

8. Serve:

Serve the Rhubarb Upside-Down Cake warm, at room temperature, or cold.

Serve with ice cream or whipped cream, if desired. Enjoy!