Rhubarb Upside-Down Cake

Ingredients

• Frosting (crumb topping):
• 4 tablespoons unsalted butter, melted
• ½ cup all-purpose flour
• ¼ cup sugar
• Coarse salt
• Cake:
• 1 ½ sticks unsalted butter, room temperature, plus more for buttering pan
• 1 pound rhubarb, trimmed and cut on a sharp diagonal about ½ inch thick
• 1 ¾ cups sugar
• 1 ½ cups flour
• 1 ½ teaspoons baking powder
• Coarse salt
• ½ teaspoon finely grated orange zest plus 1 tablespoon fresh orange juice
• 2 large eggs
• 1 cup sour cream

Directions

1. Preheat oven to 350 degrees. Make the topping by stirring together butter, flour, sugar, and ¼ teaspoon salt until moist and crumbly.
2. Make the cake: Butter a 9-inch square pan. Dot with 4 tablespoons butter cut into pieces. Toss rhubarb with ¾ cup sugar and let stand for 2 minutes. Toss again and spread in the pan.
3. Whisk together flour, baking powder, and 1 ½ teaspoons salt. Beat the remaining stick of butter and cup of sugar with a mixer on medium speed until pale and fluffy. Beat in zest and juice. Beat in eggs, 1 at a time, until incorporated.
4. Beat the flour mixture in 3 additions, alternating with sour cream, until smooth. Spread evenly over the rhubarb. Crumble the topping evenly over the batter.
5. Bake until a toothpick inserted into the center comes out clean and the top springs back when touched, about 1 hour. Let cool for 10 minutes. Run a knife around the edge of the cake and invert onto a wire rack. Let cool completely.