Rhubarb Crumb Cake

INGREDIENTS

• :
• 1/3 cup granulated sugar
• 1/3 cup dark brown sugar
• ½ teaspoon ground cinnamon
• ¼ teaspoon ground ginger
• Pinch salt
• 8 tablespoons unsalted butter, melted and still warm
• 1¾ cups cake flour
• 1½ cups small-diced rhubarb
• 2 tablespoons granulated sugar
• 1 teaspoon cornstarch
• 2 teaspoons all-purpose flour
• 1¼ cups cake flour
• ½ cup granulated sugar
• ¼ teaspoon baking soda
• ¼ teaspoon table salt
• 6 tablespoons unsalted butter, cut into 6 pieces, softened but still cool
• 1 egg
• 1 egg yolk
• 1 teaspoon vanilla extract
• 1/3 cup buttermilk
• Powdered sugar, for dusting