Preheat oven to 350 degrees F. Liberally grease two 9" cake pans with cooking spray. Set aside.
In a large bowl, beat the cake mix, buttermilk, oil, eggs and dry pudding mix with an electric mixer for 2 minutes or until blended. Pour the mixture evenly among the cake pans. Bake for approximately 20-25 minutes or until the cake springs back lightly when touched and a toothpick inserted near the center comes out clean. Cool completely.
To make the buttercream: In the bowl of a stand mixer, cream together the butter, Bailey's and vanilla until creamy, about 1 minute. Gradually add the powdered sugar, about one cup at a time, until frosting is light and fluffy. You may not use all of the powdered sugar.
To assemble the cake, place one cake layer top-side down onto a plate. Spread with 3/4th of the Bailey's buttercream, spreading out to the edges. Top with the second cake layer, top side down and even out the filling to compensate for the gap between layers.
In a medium bowl, microwave the chocolate chips and heavy cream on HIGH heat for about 45 seconds. Stir until completely smooth and pour the mixture over the top of the cake, spreading out to the edges so it drips over. Let set for about 5 minutes.
Pipe the remaining Bailey's buttercream around the perimeter of the cake and sprinkle on the chocolate sprinkles, if desired. Store cake covered at room temperature or in the fridge.