Use your 9" cake pan as a stencil to cut out two circles of parchment paper. Line your pans with the paper, then butter them.
Sift together flour, baking powder, baking soda, and salt in a medium bowl.
In a large mixing bowl (I used my stand mixer), beat butter and sugar at medium speed until pale and fluffy, then beat in vanilla.
Add eggs, one at a time, beating well and scraping down the bowl after each addition.
On low, beat in buttermilk until just combined.
Add flour mixture in three batches, mixing until each addition is just Incorporated.
Spread batter evenly in cake pan, then drop on the counter several times to get rid of the air bubbles. Be prepared for the batter to spit back at you. Lesson learned on my part.
Bake for 35-40 minutes, until golden and a wooden pick inserted in center of cake comes out clean.
Cool in the pan on a rack for 10 minutes, then run a knife around edge of pan. Invert onto rack, throw away parchment paper, then cool completely, about 1 hour.
For the frosting
Melt the chocolate in a microwave safe dish, for approximately 10-20 second intervals, stirring after each round, until fully melted.
Once the chocolate is cooled, place all ingredients into a food processor and pulse to incorporate, then blend until the frosting is smooth. Since my food processor isn't big enough, I did this this step in my stand mixer with the paddle attachment. It probably would have turned out better had I used a food processor.