Preheat oven to 350F. Grease and flour two 6" cake rounds and line with parchment.
In a medium bowl, whisk flour, baking powder, and salt until well combined. Set aside.
In a small bowl, whisk brown sugar and cinnamon. Set aside.
Using a stand mixer fitted with a paddle attachment, cream butter and sugar on med-high until pale and fluffy (approx 3mins). Reduce speed and add eggs one at a time fully incorporating after each addition. Add vanilla.
Alternate adding flour mixture and milk, beginning and ending with flour (3 additions of flour and 2 of milk). Fully incorporating after each addition.
Pour 1/4 of the cake batter (about 200g) into one of the cake pans, spread evenly. Sprinkle with half the cinnamon sugar mixture (2 1/2 Tbsp). Dollop another 1/4 of the cake batter over the top and spread as best you can (try not to life your spatula too much). Swirl through with a bamboo skewer. Repeat with next cake pan.*
Bake for 30-35mins or until a toothpick inserted into the center comes out mostly clean.
Place cakes on wire rack to cool for 10mins then turn out onto wire rack to cool completely.
Cream Cheese Frosting:
Using a stand mixer or hand mixer beat butter until pale and creamy.
Reduce speed to low. Add powdered sugar 1 cup at a time and mix on low until well blended. Increase speed to medium and beat for 3 minutes. Add vanilla and salt and continue to beat on medium for 1 minute.
Slowly add chilled cream cheese one cube at a time (about 1 tablespoon at a time). Beat well until blended and ensure no lumps of cream cheese remain.
If needed, add cornstarch or meringue powder 1 tablespoon at a time to stiffen the frosting (I didn't but this is an option).
Run mixer on low for a couple of minutes to remove any air bubbles.
Cinnamon Sugar Glaze:
Place all ingredients into a small bowl or measuring cup. Stir until smooth. Add more milk or sugar as needed to get a very thick but pourable glaze. Place plastic wrap directly on top until ready to use.
Place one layer of cake on a cake stand or serving plate. Top with approximately 1 cup of buttercream and spread evenly. Repeat with cinnamon roll layer** and place final layer of cake on top. Do a thin crumb coat all over the cake and chill for 20mins.
Frost the top and sides of the cake and smooth with a bench scraper. Swirl the top and sides using an offset spatula. Chill to set frosting, at least 30mins.
Place the trimmed tops of the two-bite cinnamon buns along the bottom if desired. Use extra buttercream to adhere them if needed.
Using a teaspoon, place dollops of glaze around the top edges of the cooled cake, allowing some to drip down. Place a few whole two-bite cinnamon rolls on top if desired.
* I just did both cake pans at the same time but it was easier to describe one at a time. If it's easier, divide the cake batter in half, before dividing it in half again for each pan. A scale helps with this.** See blog post for details on how to modify the recipe if you want to skip the cinnamon roll layer.