For the crumb topping:
3-1/2 cups cake flour (not self-rising)
2/3 cup granulated sugar
2/3 cup packed dark-brown sugar
1-1/2 teaspoons ground cinnamon
2-1/2 sticks unsalted margarine, melted
For the cake:
1-1/2 sticks unsalted margarine, room temperature
2-1/2 cups cake flour (not self-rising)
1/2 teaspoon baking soda
1 cup granulated sugar
2 large eggs plus 2 large egg yolks
1 teaspoon pure vanilla extract
2/3 cup low-fat soy milk
¾ tablespoon white vinegar
1 cup store-bought jam (optional)
Confectioners’ sugar, for dusting
Make the crumb topping: Mix together flour, sugars, cinnamon, and 1/2 teaspoon salt in a medium bowl. Pour warm melted margarine over mixture, and mix using your hands until medium to large clumps form.
Preheat oven to 325 degrees. Make the cake: Grease a 9-by-13-inch baking pan. Whisk together flour, baking soda, and 1/2 teaspoon salt in a medium bowl.
Beat margarine and granulated sugar with a mixer on medium speed until pale and fluffy, about 2 minutes. Beat in eggs and yolks, 1 at a time, then vanilla. Mix soy milk with vinegar in a small bowl. Beat in flour mixture in 3 additions, alternating with soy milk, beginning and ending with flour. Continue to beat until well combined.
Spoon batter into pan, and spread evenly using an offset spatula. Spread with jam. Sprinkle crumb-topping mixture evenly over top.
Bake until golden brown and a toothpick inserted into the center comes out clean, about 1 hour. Transfer pan to a wire rack. Let cake cool slightly, about 15 minutes. Dust with confectioners’ sugar. Serve warm or at room temperature.