Make the filling/streusel: Cut butter into the dry ingredients (brown sugar through cinnamon) with a fork, pastry cutter, or food processor until a crumb forms and set aside.
Preheat oven to 350 F. In a medium bowl, mix cake batter’s flour, baking soda, and salt. In a large bowl with an electric mixer, cream butter, brown sugar, white sugar, and vanilla until lightened in color. Add half the dry ingredients to the bowl. When mostly combined, add apple cider and apple cider vinegar. Once combined, mix in remaining dry mixture. Finish off with a spatula to avoid overmixing.
Spoon/pour half the cake batter into a greased 8x8-inch (or 9-inch round) baking dish, covering all corners with batter. Sprinkle a thin layer of streusel crumbs over the batter, then pour remaining batter on top, smoothing out to the edges of the dish. Sprinkle remaining streusel in an even layer across the top. Place on a cookie sheet on the middle rack of the oven and bake 55 to 65 minutes — if streusel crumble darkens too quickly, lightly cover with aluminum foil. Cake is done when toothpick inserted in center comes out clean. Cool completely before cutting and serving.
NOTE: Did you know the combination of 1 teaspoon baking soda and 1 tablespoon vinegar can be used in place of an egg (as a leveling agent) in many baked goods?
Per serving: 408 calories, 11 grams fat (6 grams saturated fat), 27 milligrams cholesterol, 318 milligrams sodium, 74 grams total carbohydrates, 2 grams fiber, 4 grams protein.
