Preheat the oven to 350F. Line two, 8-inch cake pans with parchment paper and spray with non-stick spray; set aside.
Place butter, eggs, vanilla, and sugar into a mixer bowl. Mix on high speed until smooth and uniform. In separate bowl, combine flour, baking powder, salt and cocoa powder. Use sifter or fine mesh strainer and sift in the flour mixture into the batter gradually, alternating with the milk; use spatula and mix by hand as to not over mix!
Divide cake batter evenly between the two pans and bake in preheated oven for approximately 30 minutes, until tops are completely set and toothpick inserted into the center comes out clean. Remove cakes from pans once baked and let cool completely on wire rack. Once cooled, slice layers in half, to have four layers total.
For Pistachio Buttercream:
Prepare the buttercream. Place pistachios into a food processor and pulse until nuts resemble fine crumbs. Add Irish cream and a few drops of green food coloring. Continue mixing in food processor for a few minutes until mixture resembles thick paste.
Into a mixer bowl, add the butter and cream cheese. Mix until well-combined and fluffy. Add the prepared pistachio cream; gradually add the powdered sugar. Add less or more to your liking; more for a thicker buttercream. Reserve about 1 1/2 cups of buttercream in a pastry bag tipped with a star tip.
Assembling the Cake:
To assemble the cake: lightly drizzle each layer with Irish cream and fill generously with buttercream. Frost the outside and smooth out top and sides. Place cake into the refrigerator and let it set for about 20 minutes. Meanwhile, prepare the chocolate ganache. Whisk together 3/4 cup chocolate chips with hot heavy cream until smooth. Pour into a ziplock bag and let the ganache stand for about 5 minutes to allow the air bubbles to dissipate.
Pour ganache over the top of the cake and use spatula to smooth out the top. Let the chocolate ganache set in the refrigerator, then garnish top with reserved buttercream. Can top cake with whole pistachios and chopped chocolate truffles.
Let cake stand in refrigerator for a few hours to set. Remove about 30 minutes before serving; perfect with coffee and tea.