Key Lime Layer Cake

INGREDIENTS
One boxed yellow cake mix (with eggs, oil and water to make according to package)
1 cup of key lime juice
1 can (14 oz) of sweetened condensed milk
1 egg
2 sticks (8 oz.) of salted butter
2 sticks (8 oz.) of unsalted butter
3 cups of powdered sugar
2 teaspoons of clear vanilla
1/4 cup crushed graham crackers
DIRECTIONS (make filling 1st to save time)
Cake
Make cake mix according to package and bake in two 8 or 9 inch round pans.
Turn cakes out of pans onto a cooling rack and cool completely.
Filling
In a medium mixing bowl blend together key lime juice, sweetened condensed milk and egg until combined.
Pour into a small casserole dish and bake at 325F degrees for 15-20 minutes until center is set.
Cool completely (I make this 1st and place it in the fridge while I make the cake and frosting)
Frosting
In a large mixing bowl, add softened butters, powdered sugar and vanilla.
Beat on high for several minutes until frosting is light and very fluffy.
Putting the cake together
Trim the tops of cakes to make them flat using a large serrated knife.
Cut each cake into 2 layers using the same serrated knife.
Place 1st layer on a serving plate and lightly layer with frosting.
Spread 1/3 of the cooled key lime filling over layer
Place 2nd layer on top and repeat until last layer has been placed on top.
Frost entire cake (top and sides) with buttercream frosting.
Crush a few graham crackers and gently press the crumbs along the sides of the cake.
Decorate as desired. I used little dollops of extra frosting and little lime green chocolates.