Preheat oven to 325F. Grease and flour three 6" cake rounds and line with parchment.
In a medium bowl, whisk flour, baking powder, and salt until well combined. Set aside.
Using a stand mixer fitted with a paddle attachment, cream butter and sugar on med-high until pale and fluffy (approx 3mins).
Reduce speed and add eggs one at a time fully incorporating after each addition. Add vanilla.
Alternate adding flour mixture and buttermilk, beginning and ending with flour (3 additions of flour and 2 of milk). Fully incorporating after each addition.
Fold in chocolate chips and mix until just incorporated.
Bake for 35-40mins or until a toothpick inserted into the center comes out mostly clean.
Place cakes on wire rack to cool for 10mins then turn out onto wire rack. Allow to cool completely.
Whipped Chocolate Buttercream:
In a large bowl, combine sifted powdered sugar and cocoa powder. Whisk to combine and set aside.
Using a stand mixer, beat butter until smooth. Reduce speed and slowly add powdered sugar & cocoa mixture one cup at a time. Add 2-4 Tbsp of cream until desired consistency is reached (I used all 4). Mix on medium until smooth (3 mins).
Add melted chocolate and mix until well combined and smooth.
Place one layer of cake on a cake stand or serving plate. Top with approximately 2/3 cup of buttercream. Repeat with remaining layers and crumb coat the cake. Chill for 20mins.