Chocolate Babka

To prepare filling, combine first 4 ingredients in a bowl, stirring with a whisk.

To prepare streusel, combine 1 tablespoon filling, powdered sugar, white rice flour, and 1/8 teaspoon salt in a small bowl; cut in 1 tablespoon butter with a pastry blender or 2 knives until mixture resembles coarse meal. Cover and refrigerate.

To prepare dough, dissolve yeast and 1 tablespoon granulated sugar in milk in a bowl; let stand 5 minutes.

Combine melted butter, oil, vanilla, egg, and egg yolk in a medium bowl; stir with a whisk. Weigh or lightly spoon flours, potato starch, and cornstarch into dry measuring cups; level with a knife. Combine flours, potato starch, cornstarch, 4 tablespoons sugar, xanthan gum, baking powder, and 1/2 teaspoon salt in a large bowl; beat with a mixer at medium speed. Add yeast mixture and egg mixture; beat at low speed until blended. Add 1/3 cup streusel; beat just until blended.

Spoon one-third of dough evenly into bottom of a 9 x 5-inch loaf pan coated with cooking spray. Top with half of filling. Repeat procedure ending with dough. Swirl dough together using the tip of a knife. Cover with plastic wrap coated with cooking spray, and let rise in a warm place (85°), free from drafts, 1 hour or until dough reaches top of pan.

Preheat oven to 350°.

Sprinkle top of dough with streusel. Bake at 350° for 45 minutes or until browned or loaf sounds hollow when tapped. Cool 15 minutes in pan on a wire rack; remove from pan. Cool completely.