In a mixing bowl, add butter, brown sugar, flour and Fruity Pebbles. Mix on medium until small a crumb starts to form. Turn mixer off.
Stir the remaining crumbs with a spatula or by hand.
Place in a freezer bag and freeze while preparing the cake.
Preheat oven to 350 degrees. Spray a 11x7 cake pan with non-stick cooking spray. This recipe will fill the cake pan completely full. You can also use an 9x13 cake pan if you're more comfortable. Add a piece of tin foil underneath the pan, just in cake any spillage occurs. (It shouldn't)
Add the milk and Fruity Pebbles in a bowl. Let it sit for 5 - 10 minutes until the milk turns make and the cereal gets soft. Meanwhile, prepare the rest of the cake.
In a large mixing bowl, add in the cooled, melted butter, vegetable oil and sugar. Mix on medium until incorporated. Scrape down the sides of the bowl.
Add in the egg and vanilla. Mix on medium until combined. Scrape the sides of the bowl.
In a separate bowl, add in the flour, baking soda and baking powder. Whisk until incorporated.
Add half of the flour mixture into the butter and sugar. Mix on medium until just combined. Scrape down the sides.
Add in all of the buttermilk and cereal mixture. Mix on medium until combined. Scrape down the edges.
Add the remaining flour and mix on medium until incorporated.
Spread the batter into the 11x7 cake pan. Make sure it goes to all edges and corners.
Remove crumb from the freezer and pile it all on the top and spread it to all the edges and corners.
Place in the oven and bake for 53 minutes to 1 hour. To check to see if the coffee cake is done, place a clean toothpick into the center of the cake. If it comes out with batter on it, it needs more time. If it comes out clean or with moist crumbs, it's done.