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1
Grease and lightly flour two 9x1-1/2-inch round cake pans (or three if you have three). Set pans aside.
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2
In an extra-large mixing bowl, combine flour, sugar, baking soda, salt, and baking powder. Add milk, sour cream, melted chocolate, butter, eggs, and vanilla.
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3
Beat with an electric mixer on low speed about 1 minute or until mixture is combined. Beat on high speed 3 minutes, scraping sides of bowl occasionally.
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4
Place 1/3 of the batter in each of the cake pans. (I only had two, so I had to wait to bake the last layer.).
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5
Bake in a 350 degree F oven for about 25 minutes or until a toothpick inserted near the center comes out clean.
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6
Cool cakes in pans on wire racks for 10 minutes.
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7
Remove cakes from pans. Cool layers thoroughly on wire racks.
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8
Prepare Cookie Filling: In large mixing bowl, combine powdered sugar, butter, milk, and vanilla. Beat with electric mixer on low speed until combined; beat on medium speed until very smooth.
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9
Beat in enough additional milk (1 to 2 tablespoons) until frosting is easy to spread. Reserve 3/4 cup of the filling (without crumbs) for the cake garnish.
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10
Stir crushed sandwich cookies into remaining mixture.
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11
To assemble the cake, put a cake layer on a serving plate and top with half of the crumb-filled cookie filling, spreading filling evenly over layer. Add a second layer of cake and spread with the remaining portion of the cookie filling. Top with remaining cake layer.
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12
Prepare the Chocolate-Sour Cream Frosting: In a 4-cup Pyrex measuring cup (or a small saucepan), melt 1 cup semisweet chocolate pieces and 1/4 cup butter, stirring frequently. Cool about 5 minutes. Stir in sour cream.
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13
Gradually add powdered sugar, beating until smooth and easy to spread.
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14
Spread top and sides of cake with frosting.
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15
Spoon the reserved cookie filling onto the center top of the cake. Carefully spread filling to force some down sides of cake. Garnish the top of the cake with the 4 crushed chocolate sandwich cookies.
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16
Cover and store in the refrigerator.