Step 1Sift together flour, baking powder, baking soda and salt. Set aside.
Step 2Beat butter and brown sugar at medium speed until light and fluffy, about 2 minutes. Scrape down bowl; mix in chocolate until well combined. Add eggs, 1 at a time, mixing each well before adding another.
Step 3On lowest speed, add dry ingredients and buttermilk alternately, beginning and ending with dry ingredients and mixing well before each addition. After last addition, scrape down bowl. Mix until well combined. Transfer to greased 10-inch springform pan that has been lined with greased circle of parchment paper. Set pan on baking sheet. Smooth cake top with spatula.
Step 4Bake at 375 degrees until toothpick inserted in center comes out clean (top gets very dark), about 40 minutes. Cool on rack. Use small flexible knife to release cake from sides of pan. Remove sides. If cake top is too dark, carefully trim and discard top surface. Invert cake onto platter. Remove springform bottom and paper.
Step 1Bring cream to boil in small saucepan. Place chocolate in bowl and pour cream over top. Stir with wooden spoon until smooth. Pour over cake. Use metal spatula to smooth top and evenly coat sides of cake. (Can be made a day ahead and kept at room temperature.)