3 cups rhubarb, chopped into 1-inch pieces

3/4 cup granulated sugar (for rhubarb)

4 tbsp salted butter (plus more for greasing pan)

1/2 cup salted butter, softened

1 cup + 2 tbsp granulated sugar (for cake)

2 large eggs, at room temperature

1 tsp orange zest

1 tbsp + 1 tsp orange juice

1 tsp vanilla extract

1/2 cup full-fat sour cream

1 and 1/2 cups + 2 tbsp all-purpose flour

1 and 1/2 tsp baking powder

1/4 tsp salt

1/4 cup 2% milk, at room temperature

1/4 cup salted butter, melted (for crumb layer)

1/2 cup + 1 tbsp all-purpose flour (for crumb layer)

2 tbsp granulated sugar (for crumb layer)

2 tbsp packed brown sugar

2 tsp raw turbinado sugar (optional)

Pinch of salt (for crumb layer)

Pinch of cinnamon (for crumb layer)

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Preheat oven to 350℉. Grease a 9-inch cake pan well. Place parchment round in the bottom and butter over it. Scatter 4 tbsp butter chunks in pan. Set aside.
Mix chopped rhubarb with 3/4 cup sugar. Let sit to release juices.
In a bowl, cream 1/2 cup butter for 1 minute. Add sugar and beat until fluffy. Add eggs one at a time, then mix in orange zest, juice, vanilla, and sour cream until smooth.
Mix flour, baking powder, and salt. Add to wet ingredients on low speed. Add milk and mix until just combined.
Spread rhubarb evenly in prepared pan. Pour batter over rhubarb and smooth the top.
Make crumb layer: Mix melted butter with crumb ingredients until clumpy. Sprinkle over cake batter.
Bake for 45–55 minutes or until a toothpick comes out with moist crumbs. Let cool in pan 10 minutes.
Run a knife around edge and invert onto a wire rack with a baking sheet underneath. Cool completely before serving.