I've converted many a cake-hater with this little number (unless you hate peanut butter, then you're out of luck:)). I tried chocolate/peanut butter cake for the first time at Hershey's Chocolate World and I've worked on this recipe for over a year trying to re-create it. It's fabulous! I've never had such a moist, chocolate-y cake.
Peanut Butter Chocolate Fudge Cake
Cake: 
1 box Chocolate Fudge cake mix (or any chocolate cake mix)
1 (3 oz.) box instant chocolate pudding
2 C. sour cream
5 eggs
1 C. melted butter
1 tsp. almond extract
1 C. chocolate chips (I recommend ghirardelli)
In a large bowl, beat first 6 ingredients until well blended. Mixture will be thick. Stir in chocolate chips. Pour into three greased 9” pans. Bake at 350 degrees for 25-40 minutes. (Keep an eye on them so they don’t overbake). Cool completely. I actually like to pop them in the freezer for easy assembling.
1 box Chocolate Fudge cake mix (or any chocolate cake mix)
1 (3 oz.) box instant chocolate pudding
2 C. sour cream
5 eggs
1 C. melted butter
1 tsp. almond extract
1 C. chocolate chips (I recommend ghirardelli)
In a large bowl, beat first 6 ingredients until well blended. Mixture will be thick. Stir in chocolate chips. Pour into three greased 9” pans. Bake at 350 degrees for 25-40 minutes. (Keep an eye on them so they don’t overbake). Cool completely. I actually like to pop them in the freezer for easy assembling.
Peanut Butter Buttercream Icing
Ingredients
2/3 cup solid vegetable shortening
2/3 cup butter
1 3/4 cup peanut butter
1 cup milk
2 teaspoons Vanilla Extract
6-8 cups sifted confectioners' sugar (approx. 2 lb.)
Directions
Cream shortening and butter with electric mixer.
Add peanut butter, milk and vanilla.
Gradually add 6 c. sugar, one cup at a time, beating well on medium speed. Scrape sides and bottom of bowl often. When all sugar has been mixed in, icing will appear dry.
Add milk and beat at medium speed until light and fluffy. Add more sugar (or milk) as needed until icing is desired consistency (should be firm enough to hold a shape but still smooth)
Keep icing covered with a damp cloth until ready to use. For best results, keep icing bowl in refrigerator when not in use. Refrigerated in an airtight container, this icing can be stored 2 weeks. Re-whip before using.
Ingredients
2/3 cup solid vegetable shortening
2/3 cup butter
1 3/4 cup peanut butter
1 cup milk
2 teaspoons Vanilla Extract
6-8 cups sifted confectioners' sugar (approx. 2 lb.)
Directions
Cream shortening and butter with electric mixer.
Add peanut butter, milk and vanilla.
Gradually add 6 c. sugar, one cup at a time, beating well on medium speed. Scrape sides and bottom of bowl often. When all sugar has been mixed in, icing will appear dry.
Add milk and beat at medium speed until light and fluffy. Add more sugar (or milk) as needed until icing is desired consistency (should be firm enough to hold a shape but still smooth)
Keep icing covered with a damp cloth until ready to use. For best results, keep icing bowl in refrigerator when not in use. Refrigerated in an airtight container, this icing can be stored 2 weeks. Re-whip before using.
TO ASSEMBLE CAKE: 
First of all, if you don't have a canvas decorating bag and an extra-large star tip, I recommend getting one (you can get them at Wal-Mart, under $5 for all). The tip is Wilton size 1M. I use it for EVERYTHING. It makes super-fast, gorgeous cupcakes and is the very easiest way to decorate a cake (no crumbs!).
First of all, if you don't have a canvas decorating bag and an extra-large star tip, I recommend getting one (you can get them at Wal-Mart, under $5 for all). The tip is Wilton size 1M. I use it for EVERYTHING. It makes super-fast, gorgeous cupcakes and is the very easiest way to decorate a cake (no crumbs!).
Place a small dab of frosting on a cake platter, set first cake layer on top (it's easiest to assemble if the layers are frozen but not necessary). Pipe out (or spread) frosting over top of layer and add next layer. Repeat with final layer. Spread a layer of frosting on the very top and sprinkle chocolate shavings on top (or chocolate chips or chopped reeses peanut butter cups!).
Using your large star tip, make stripes up and down the sides of cake. This is super-easy and you have no frosting crumbs! Hooray!
To finish, draw "S" shapes all around the top edge of the cake. Voila! You have decorated a professional looking cake in 5 minutes! Enjoy! 
