Place 1 1/2 cups all-purpose flour, 1 cup packed light brown sugar, 2 teaspoons ground cinnamon, and 1/2 teaspoon kosher salt in a large bowl and whisk to combine.
Cut 12 tablespoons cold unsalted butter into small pieces. Add to the bowl. Use a pastry cutter, fork, or fingers to cut or rub the butter into the dry ingredients. Take care to mix the dry ingredients from the bottom of the bowl up to the top. Continue cutting or rubbing until the largest pieces of butter are the size of peas and the flour is no longer dry-looking, 3 to 5 minutes. The mixture should resemble damp sand.
Refrigerate while you make the cake. (If you only have 1 large bowl, transfer the topping to a smaller bowl so you can reuse the large bowl for the cake.)
Arrange a rack in the middle of the oven and heat the oven to 350°F. Coat a 9x13-inch baking dish with cooking spray.
Cut 2 sticks unsalted butter into 16 pieces. Place in a large microwave-safe bowl. (No need to rinse the bowl first if reusing the topping bowl.) Microwave until melted, 1 to 1 1/2 minutes. (Alternatively, melt on the stovetop and transfer to a large bowl.)
Add 1 1/2 cups granulated sugar and whisk until combined. Add 3 large eggs one at a time, whisking well after each addition. Add 1 1/2 cups sour cream and 2 teaspoons vanilla extract and whisk to combine. Add 3 cups all-purpose flour, 2 teaspoons baking powder, 1/2 teaspoon kosher salt, 1/2 teaspoon ground nutmeg, and 1/4 teaspoon baking soda. Gently whisk the dry ingredients together on top of the wet. Switch to a flexible spatula and gently fold together until combined and no pockets of flour remain (batter will be very thick and slightly lumpy).
Dollop half the batter evenly into the baking dish (about 2 1/2 cups or 24 ounces). Use an offset spatula to smooth the batter into an even layer, going all the way to the edges. Sprinkle 1 1/2 cups of the topping evenly over the batter. Dollop the remaining batter evenly over the filling and spread into an even layer. Pick up handfuls of the remaining topping and squeeze it together. Crumble it evenly over the cake and repeat until all the topping is used up.
Bake until the center of the cake is fully risen and springs back when lightly pressed, 50 to 55 minutes. Let cool for at least 1 hour.
If making the icing, place 1 cup powdered sugar and 2 tablespoons milk in a small bowl and whisk until smooth. Drizzle over the cake. Let the icing set for a few minutes before cutting the cake and serving.
