Preheat the oven to 350°F. Grease a 9×13 pan and line it with parchment paper, leaving a slight overhang for easy removal. Set aside.
Make the cake batter by whisking together the flour, baking powder, baking soda, and salt in a medium bowl. Set aside.
2½ cups all-purpose flour, 1 tsp baking powder, ½ tsp baking soda, ½ tsp salt
In the bowl of a stand mixer, cream the butter and sugar together for 2–3 minutes, until light and fluffy.
1 cup salted butter, softened, 1¾ cups granulated sugar
Add the eggs one at a time, mixing until incorporated. Mix in the vanilla extract and then add the sour cream, mixing until smooth.
3 large eggs, room temp, 2 tsp vanilla extract, 3/4 cup sour cream
With the mixer on low, gradually add the dry ingredients and then slowly add the milk until just combined. Set the batter aside and make the crumb topping and cinnamon layer.
½ cup milk
Make the crumb topping and cinnamon layer by using a pastry cutter or food processor to combine the flour, both sugars, ½ Tbsp cinnamon, and salt. Cut in the cold butter until the mixture forms pea-sized crumbs.
1 1/4 cup all-purpose flour, 1 cup light brown sugar, packed, ½ cup granulated sugar, ½ tsp salt, 12 Tbsp cold butter, cubed, 1/2 Tbsp ground cinnamon
Remove ¾ cup of the crumb topping and stir in the remaining 1 Tbsp of cinnamon. This will be your middle cinnamon layer.
1 Tbsp ground cinnamon
Spread a little more than half of the cake batter into the bottom of the prepared pan. Sprinkle the cinnamon crumb layer evenly over the batter.
In small spoonfuls, drop the remaining batter over the cinnamon layer. Gently spread the batter to connect the dollops and cover the cinnamon layer as evenly as possible (it’s okay if it’s not perfect).
Sprinkle the crumb topping evenly over the top of the cake.
Bake at 350°F for 40–50 minutes, or until a toothpick comes out with moist crumbs. If the top begins to brown too quickly, loosely tent the pan with foil.
Whisk the glaze ingredients together and drizzle over the top of the cake immediately after removing it from the oven.
4 Tbsp salted butter, melted, 1½ cups powdered sugar, 2-3 Tbsp milk, 1 tsp vanilla extract
Serve warm or cooled. Enjoy! Store in an airtight container at room temperature for up to 4 days.