• Preheat the oven to 350℉/180℃. Grease the entire pan, then line the bottom of a 9-inch round cake pan with parchment paper. Alternatively, use an 8x8 inch square pan line with parchment paper.

Rhubarb Filling

• Wash and dice the rhubarb into 1cm pieces.
  3 cups diced rhubarb
• Add the rhubarb, lemon juice, and sugar to a medium saucepan over medium heat and cook for about 3 minutes until the rhubarb begins to soften slightly.
  2 tablespoons lemon juice, ¼ cup granulated sugar
• Mix together the cornstarch and water and stir it into the rhubarb. Cook for thirty seconds, stirring the whole time.
  1 tablespoon corn starch, 2 tablespoons water
• Remove the pan from the heat and stir in the vanilla. Set aside while you make the streusel and cake batter.
  1 teaspoon vanilla paste or extract

Crumb Topping

• Melt the butter, then use a fork to mix it together with the flour, sugars, and salt until medium-sized crumbles form. Set aside.
  6 tablespoon unsalted butter, ¾ cup + 2 tablespoons all-purpose flour, ½ cup granulated sugar, 2 tablespoons light brown sugar, ¼ teaspoon kosher salt

Cake

• Whisk together the flour, baking powder and salt and set aside.
  1½ cups all-purpose flour, 1 teaspoon baking powder, ½ teaspoon kosher salt
• In the bowl of a stand mixer, cream together the butter and sugars on medium high speed for five minutes, then scrape down the sides.
  6 tablespoons unsalted butter, ½ cup light brown sugar, ½ cup granulated sugar
• Add the eggs, oil, vanilla and mix on medium speed for one minute. Scrape down the sides again.
  ¼ cup vegetable oil, 2 large eggs, 2 teaspoons vanilla paste or extract
• Mix together the sour cream and milk. With the mixer running on low, add half of the flour mixture then half of the milk mixture. Repeat with the remaining wet and dry ingredients.
  ¼ cup full-fat sour cream, ¼ cup whole milk
• Mix until the flour is just combined, and use a rubber spatula to finish folding in any flour on the sides of the bowl.

Assembly

• Add the cake batter to the prepared pan and spread it to meet the edges.
• Spread the rhubarb filling evenly over the top of the cake batter.
• Sprinkle the crumb over the top, getting it the whole way to the edges.
• Bake the cake in the center rack of the preheated oven for 45-55 minutes until the center is no longer jiggly and a toothpick inserted down into the cake layer comes out clean.
• Place the pan on a wire rack to cool for at least 30 minutes, before slicing. Dust with powdered sugar, if desired.