Preheat the oven to 350 degrees F (175 degrees C). Grease and flour two 9-inch round cake pans.

Melt butter and unsweetened chocolate together in a saucepan over low heat, stirring constantly, about 5 minutes.

Combine flour, sugar, baking soda, salt, and cocoa powder in a large bowl of an electric mixer. Add melted chocolate mixture, buttermilk, and eggs. Beat on low speed for 1 minute; increase speed to high and beat until light and fluffy, about 2 minutes. Divide batter between the prepared cake pans.

Bake in the preheated oven until cakes spring back when gently pressed, 25 to 30 minutes. Cool in the pans for 5 minutes; invert onto a wire rack to cool completely.

Meanwhile, bring cream just to a boil in a saucepan. Remove from heat and stir in chocolate chips; mix until frosting is smooth and starts to thicken.

Place 1 cooled cake layer on a cake plate. Spread with 1/3 cup frosting. Top with second layer. Frost the top and sides with remaining frosting.