Preheat oven to 350°.
In a mixer bowl, add the cake mix with eggs, 1/2 cup softened butter and milk.
Mix at low speed until combined, raise to medium speed, 2 minutes. Scrape down sides and mix another minute.
Reserve 1 1/2 cups cake batter. Pour the remaining batter into a buttered 15 x 10 x 1 jelly roll pan and spread evenly.
Melt 1 cup chocolate chips with 1 tablespoon of sour cream over low heat in a pan or microwave in a safe bowl in 30 second intervals until smooth.
Pour melted chocolate into reserved cake batter. Mix to combine.
Dollop chocolate batter over yellow cake batter. Swirl with a butter knife to create marbling.
Bake for 17- 20 minutes or until a toothpick inserted in center comes out clean.
Cool cake on a wire rack.
Prepare the icing by melting remaining 1 cup chocolate chips with 1/4 cup butter in a small pan over low heat or in a microwave in 30 second intervals. Stir until chocolate is melted and smooth.
In a mixer bowl, add 1/2 cup sour cream, vanilla extract, melted chocolate and powdered sugar.
Mix at low speed just until combined, raise to medium-high speed and whip the frosting 2 minutes.
Spread frosting over completely cooled cake.
Slice and serve.