1. Prepare the Pan and Preheat:

• Preheat the oven to 350°F (175°C).
• Spray a 9-inch round cake pan with baking spray with flour. This type of spray ensures the cake won’t stick.
• Line the bottom of the pan with a circle of parchment paper. Do not spray the parchment again. The slight stickiness will help hold the rhubarb in place.

2. Prepare the Rhubarb:

• Trim the rhubarb stalks, discarding any leaves (rhubarb leaves are toxic).
• Cut the rhubarb into 2-inch diagonal pieces.
• Cut each piece in half lengthwise so that each piece has a pink side and a green-white side. This creates visually appealing pieces.
• Place the rhubarb in a large bowl.
• Sprinkle with 2 tablespoons of granulated sugar and the cornstarch.
• Toss to combine, ensuring the rhubarb is evenly coated.

3. Arrange the Rhubarb:

• Place the rhubarb in the prepared pan, pink side down. This will create the beautiful pattern on top of the cake after it’s inverted.
• Trim the pieces as needed to fit snugly in the pan, creating a single layer.

4. Make the Caramel Sauce:

• In a medium stainless-steel skillet, melt 1/4 cup of unsalted butter over medium heat.
• Stir in the brown sugar, honey, and 1/4 teaspoon of salt.
• Bring the mixture to a simmer, and cook until the sugar is melted and fragrant, about 3 minutes.
• Slowly pour in the orange juice, whisking constantly until combined and smooth.
• Pour the caramel evenly over the rhubarb in the pan.

5. Make the Cake Batter:

• In the bowl of a stand mixer (or using a hand mixer), beat the remaining 1/2 cup of softened butter, the remaining 1 cup of granulated sugar, orange zest, and vanilla extract at medium speed until creamy, 2 to 3 minutes. Scrape down the sides of the bowl as needed.
• Add the eggs, one at a time, beating well after each addition.
• In a separate medium bowl, whisk together the flour, baking powder, ginger, baking soda, and the remaining 1/2 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. Do not overmix.

6. Assemble and Bake:

• Carefully dollop the batter onto the rhubarb in the pan.

• Gently spread the batter with an offset spatula, making sure not to press too hard or move the rhubarb pieces.

• Bake for 30 minutes.

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

• Continue baking until a wooden pick inserted into the center comes out clean, 15 to 20 minutes more.

7. Cool and Invert:

• Remove the foil and let the cake cool in the pan on a wire rack for 10 minutes only. This is important; if you let it cool too long, the caramel will harden and the cake will stick to the pan.
• Run a knife around the sides of the cake to loosen it.
• Carefully invert the cake onto a serving plate.
• Remove the parchment paper.
• Let the cake cool completely, or serve it warm, at room temperature, or even cold.

8. Serve:

• Serve with ice cream or whipped cream, if desired.