Peanut Butter Fudge Cake

With a lot of snow hitting Maryland this year and a wedding to bake for in July, I've utilized some time at home to practice my cake decorating and making skills. I've had a lot of practice with cupcakes but am way behind in cake practice. I found this recipe on Pinterest and it sounded delicious.
I used a 1M tip to make the swirls. The swirls stick better if you spread a thin layer of frosting onto the cake first.

yield: one 9" cake

ingredients
for cake
2C all-purpose flour
2C sugar
1 tsp baking soda
1C butter, room temperature
¼ C cocoa powder
1C water
1/2C buttermilk
2 eggs, lightly beaten
1 tsp vanilla extract

for peanut butter frosting
½C butter, room temperature
1C creamy peanut butter
3 tbs milk
2C powdered sugar

directions

1. for cake: Preheat oven to 350°.
2. Grease and flour a round or square pan (I used a 9" spring form pan because it's what I had around).
3. In a large bowl or stand mixer, combine flour, sugar, and baking soda.
4. Melt butter in a heavy saucepan.
5. Stir in cocoa.
6. Whisk in water, buttermilk, and eggs; stirring well.
7. Cook over medium heat, stirring constantly until the mixture boils.
8. Add to flour mixture, stirring until smooth.
9. Stir in vanilla extract. 
10. Pour batter into pan.
11. Bake for 20-25 minutes or until a toothpick inserted comes out clean.
12. Cool for 10 minutes on a wire rack.
13. Cool completely before frosting.
14. for peanut butter frosting: In a large bowl or stand mixet, beat butter and peanut butter until smooth.
15. Gradually on low, add in powdered sugar.
16. When mixture starts to thicken, add milk one tablespoon at a time.
17. Beat on medium-high for at least 3 minutes until the frosting is smooth and fluffy.
18. Spread a thin layer of frosting on the cake.
19. Pipe on swirls using a bag fitted with a 1M tip. (Or just spread frosting all over the whole cake).
20. Enjoy!