Preheat the oven to 350 degrees, and line a 12-cup muffin tin. Set aside.
In a large bowl, beat the butter and sugar together until fluffy. Add in the eggs, sour cream, and vanilla. The mixture will be slightly lumpy. Add in the self-rising flour, cake flour, and salt. Mix until combined, it will be a thick batter.
Mix in the milk until combined. In a small bag or small food processor, crush the cereal until it is mostly a powder, but still has some chunks of cereal left. Fold into the prepared batter.
Scoop the batter evenly into your 12 muffin cups. Bake at 350 degrees for 16-18 minutes, or until a tooth pick comes out clean. Cool completely before frosting.
In a small saucepan, cook the flour, milk, and salt until a thick paste forms. You'll want to stir the mixture constantly while heating over medium heat. Remove from heat, and cool the mixture COMPLETELY before continuing.
In a large bowl or the bowl of a stand mixer, beat the butter and sugar for 5-10 minutes, or until the mixture is noticeably lighter in color, and the sugar has mostly dissolved.
Beat in the flour mixture, and beat until it becomes light and fluffy. It will separate during this process, just keep beating. Add in the vanilla extract, and frost cooled cupcakes generously.