Big Crumb Coffee Cake

Ingredients

• 1/2 pound rhubarb, trimmed
• 1/4 cup sugar
• 2 teaspoons cornstarch
• 1/2 teaspoon ground ginger
• 1/3 cup dark brown sugar
• 1/3 cup granulated sugar
• 1 teaspoon ground cinnamon
• 1/2 teaspoon ground ginger
• 1/8 teaspoon salt
• 1/2 cup (1 stick or 4 ounces) butter, melted
• 1 3/4 cups cake flour
• 1/3 cup sour cream
• 1 large egg
• 1 large egg yolk
• 2 teaspoons vanilla extract
• 1 cup cake flour (ditto on the all-purpose flour–worked just fine)
• 1/2 cup sugar
• 1/2 teaspoon baking soda
• 1/2 teaspoon baking powder
• 1/4 teaspoon salt
• 6 tablespoons softened butter, cut into 8 pieces.