Preheat oven to 350F. Grease and flour three 6" cake rounds, line with parchment.
In a medium bowl, whisk flour, baking powder, cinnamon, and salt and set aside.
Beat butter until smooth. Add sugar and beat on med-high until pale and fluffy (2-3mins).
Reduce speed and add eggs one at a time, fully incorporating after each addition. Add vanilla.
Alternate adding flour mixture and buttermilk, beginning and ending with flour (3 additions of flour and 2 of milk). Fully incorporating after each addition. Mix until just combined
Spread batter evenly into prepared pan and smooth the top with a spatula.
Bake for 30mins 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.
Preheat oven to 300F and line a sheet pan with parchment or a silicone mat.
Whisk all dry ingredients. Add wet ingredients and mix until small clumps form.
Spread evenly and bake for 15mins. Cool completely before using on cake.
Cream Cheese Frosting:
Beat butter and cream cheese on med-high until pale and smooth (approx. 2mins).
Reduce speed to low and add sugar 1 cup at a time. Add vanilla and pinch of salt. Increase speed to high and beat for 2-3 minutes until pale and smooth.
Place one layer of the cake onto a cake stand or serving plate. Top with 1 cup of frosting and spread evenly. Sprinkle with crumble as desired. Repeat with remaining layers. Frost outside of cake and press crushed Cinnamon Toast Crunch cereal into the sides.