Preheat the oven to 350 degrees, grease and flour three 8x2 inch round pans 

In a medium bowl, whisk the flour, cocoa, baking powder, and baking soda.  Set aside

In another bowl, add the buttermilk, Baileys Irish Cream, oil and vanilla.  Set aside

In the bowl of your mixer, add the softened butter and mix at medium speed until smooth. Gradually add the sugar and continue to mix on medium speed 3 to 5 minutes until lightened in color and fluffy.  

Add the eggs one at a time, mixing until the yellow of the yolk disappears.

Alternately, add the flour mixture and buttermilk mixture, beginning and ending with the dry ingredients (3 additions of dry and 2 of wet).

Do not mix above medium speed or over mix the batter.

Bake at 350 degrees for 25 to 30 minutes or until a toothpick inserted in the center comes out clean or with just a few crumbs attached.  Let cool 5 to 10 minutes and turn out of pan.

Chilling the bowl and beaters will shorten the mixing time. Use a hand mixer a stand mixer with whisk attachment, or you can whisk by hand.

Combine the dry pudding mix, heavy cream and Baileys Irish Cream  in a mixing bowl.

Begin on low speed until everything is well combined.  Increase speed and mix until the mixture is thick.If mixing by hand, simply whisk rapidly for a couple of minutes or until mixture is thick.  

Refrigerate until ready to use.  Cakes with this filling should be refrigerated until 1 to 2 hours before serving.If you are making the filling in advance, I think it would be fine refrigerated 5 days.

Cream the softened butter until smooth.  Add approximately half of the powdered sugar, most of the milk and Irish Cream.

Mix at medium speed until the powdered sugar is incorporated.  Add remaining powdered sugar and milk.  Mix at medium speed another 3 to 5 minutes, or longer. 

Scrape the sides and bottom of the bowl occasionally.  The mixture will become very smooth.  Lower the mixing speed near the end of mixing, this will reduce air bubbles in the buttercream. 

Place your chocolate into a microwave safe bowl.

Pour heavy cream over the chocolate and place in the microwave for 1 minute. (Times may vary, see our note below).

Remove and stir. Microwave for 30 seconds more and when the chocolate has melted almost completely, allow it to sit for 1 minute more before stirring. Stir until you've reached a silky smooth consistency.

*Microwave times may vary. If you are making a smaller amount of ganache, you will reduce the microwave time. If your chocolate hasn't melted after the 1 ½ minutes recommended in the instructions, simply microwave in small 10-15 second increments until it has nearly completely melted. Let sit one minute, then stir until smooth.

Allow finished ganache to cool until it is a good consistency for dripping. (You can test this on the inside of your bowl. If it moves too quickly down, let it thicken a little more.

When it is time to assemble your cake, place the first layer on the pedestal or cake base. Pipe a ring/dam around the edge of the layer (about ¼ inch from the edge). You could apply the dam using a piping bag with the tip snipped away, or you can use a tip 12. Then, apply your mousse filling within the dam and repeat the steps.

Frost the cake with a crumb coat, chill if necessary to firm things up, then follow with final coat of frosting. I use a heated bench scraper to smooth everything out.

To apply the ganache drip, load the ganache into the piping bag (it shouldn't be hot). Snip the tip of the piping bag, add apply the drip to your *chilled* tier. Chilling the tier first helps the drips not to go all the way down. I like to add the drip around the edge of the cake first, then spread additional ganache on top.

I piped stars of Irish cream buttercream around the top edge of the cake using a 6B tip, and placed a chocolate chip into each one. I also used a tip 21 to create a piped border with a braided look around the base. Decorate however you like! ;0)