Preheat your oven to 350°F. Grease two 6-inch cake pans with cooking spray and line the bottoms with parchment paper.
Add brown sugar, flour, butter, and cinnamon to a medium sized bowl. Using your hands, mix all ingredients together until they form small clumps. You don't want to make them too big, or they will sink into the cake. Place in the freezer until the cake batter is ready.
CAKE
Sift the flour, cornstarch, baking powder, baking soda, nutmeg, and salt in a medium bowl and set aside
In the bowl of a stand mixer, cream the softened butter and brown sugar until light and fluffy (about 2–3 minutes).
Beat in the eggs, one at a time, mixing until fully incorporated.
Alternate adding the dry ingredients and the sour milk to the butter mixture, starting and ending with the dry ingredients. Mix just until combined—do not overmix. The batter should be smooth but thick.
Divide the batter evenly between the two prepared cake pans. Use a spatula to spread it evenly across the pans.
Sprinkle crumb topping evenly across the top of one of the cake layers. Make sure the topping is spread out to the edges of the pan. If it is all together, it will cause the cake to sink in the middle.
Bake for 25–30 minutes or until a toothpick inserted into the center of the cakes comes out clean and the top is golden brown.
Let the cakes cool in the pans for 10 minutes, then transfer them to a wire rack to cool completely.y
CREAM FILLING
Combine flour and milk in a small saucepan and whisk together until the flour is fully dissolved. Cook over low heat till thick, stirring constantly. Watch carefully to make sure it doesn't start to clump together. Just as it thickens, remove from heat. Transfer to a small bowl and set aside until fully cooled.
In the stand mixer bowl, use the paddle attachment, cream butter, and sugar. Add cooled milk and flour mixture and beat till fluffy.
Spread the cream filling of the cooled cakes across the bottom layer of the cake without the crumble.