Preheat oven to 350 degrees. Spray the bottom and sides of an 8" square cake pan with vegetable spray. Cut a piece of parchment paper to the dimensions of the cake pan and fit it into the bottom (this is an extra layer of protection against sticking). Set aside.
In a large bowl, beat the butter, brown sugar and granulated sugar until fluffy. Use a rubber spatula to scrape down the bowl. Add the egg, egg yolk and vanilla and beat until combined. Add the buttermilk and beat again -- mixture will look curdled, it's ok.
Sift together the dry ingredients and beat into the butter mixture on low speed until just combined. Transfer the batter into the prepared pan and use a spatula to spread it out evenly. Bake for 30-35 minutes until a cake tester comes out clean.
Cool the cake for 5-10 minutes in the pan, then invert it onto a cooling rack and cool cake completely before frosting.
For the frosting
Melt the chocolate: Place the chocolate in a small microwaveable glass bowl and microwave in short 15-20 second increments until the chocolate has melted into a glossy pool. Don't stir. Cool to room temperature.
In a large bowl combine the powdered sugar, butter, salt, cream and vanilla. Beat on medium speed until smooth and creamy. Add the room temperature chocolate and beat until well combined. Turn the beaters up to a medium high or high and beat it for a minute more.
Place the cake upside down on your cake stand or platter.
Spoon the frosting into the center of the cake and use an offset spatula to push the frosting out from the center in short little swirls to give it that dreamy icing landscape. You can frost the sides if you like -- my daughter was shocked that I didn't, but that's the way Deb did it on her site.