Preheat oven to 350°F. Line a 9x13-inch baking pan with parchment paper, and grease with cooking spray.

In a large bowl, sift or whisk together the flour, baking powder, and salt. Set aside.

In the bowl of a stand mixer, beat the butter and sugar, on high until light and fluffy, about 5 minutes. Add in the eggs, one at a time, mixing well after each addition. Stir in the vanilla extract. Add the dry ingredients, alternating between the buttermilk, starting and ending with the dry ingredients.

Combine the chocolate, cereal, brown sugar and cinnamon in a food processor and pulse until the mixture resembles fine sand.

Pour half of the cake batter into the prepared pan, spreading out into an even layer. Sprinkle the cinnamon chocolate filling evenly over the top. Then pour the remaining half of the cake batter over the top and spread out as evenly as possible. Bake until a toothpick, inserted in the middle, comes out clean, about 30 to 35 minutes. Remove from oven and let cool about 25 minutes before inverting onto a wire rack to cool completely.

To make the buttercream, beat the butter and cream cheese until light and fluffy, about 5 minutes. Beat in half of the powdered sugar and mix until just combined. Stir in the vanilla, cinnamon, and heavy cream and beat on medium until well combined. Add in the remaining powdered sugar and beat on high until light and fluffy, about 3 minutes.

Transfer the cake onto a platter or upside-down baking sheet and then dollop the buttercream on top. Smooth out with an offset spatula. Slice and serve. Leftovers can be stored wrapped in plastic and stored in the fridge for up to 3 days. Thaw out at room temperature before eating. Enjoy!