Banana Cake with Crackly Brown Sugar Frosting

I've been back in Ohio for the past week and a half visiting my parents, and this cake is one of the first things my mom and I baked together.  We were really skeptical when it came out of the oven, because the top had a lot of cracks and it just looked like it would be really dry and heavy.  Luckily, looks were deceiving, because the cake was moist and delicious.  On top of that, the frosting is absolutely to-die-for, maybe my favorite frosting ever! It's basically like a butterscotch fudge spread all over the top of the cake, and it pairs perfectly with the banana cake.  The original recipe was called "banana snack cake", which I can definitely agree with - this cake was easy to snack on! 

Banana Cake with Crackly Brown Sugar Frosting (from You Made That Dessert?)

8 T butter, plus more for pan
1-1/2 cups all-purpose flour, plus more for pan
1 cup whole-wheat flour
1 tsp baking powder
1 tsp baking soda
1/2 tsp salt
4 large, very ripe bananas
1 cup sour cream or yogurt (or a mix)
1 cup packed dark brown sugar
1 large eggs
1/2 cup finely chopped walnuts

For the frosting:
1-1/2 cups packed dark brown sugar
1/3 cup heavy cream
 5 T butter, cut into 10 pieces
1/4 tsp salt

1. Preheat the oven to 350 degrees.  Grease and flour a 9x13 baking pan.
2. In a small bowl, whisk together to flours, baking powder, baking soda, and salt.  In a large bowl, mash the bananas and then stir in the sour cream and vanilla
3. In a large bowl, beat the butter and brown sugar until fluffy with an electric mixer, about 2 minutes.  Scrape down the sides of the bowl and beat another 30 seconds.  Add the eggs one at a time, beating after each addition.
4. Stir half the dry ingredients into the butter-sugar mixture with a rubber spatula.  Stir in the banana mixture, and then the remaining flour and the walnuts.  Be careful not to over mix.
5. Pour the batter into the pan and smooth the top.  Bake 45-50 minutes, until a toothpick comes out clean.  Let cool completely before making the frosting.
6. Make the frosting: Combine all frosting ingredients in a saucepan.  Place over medium heat and cook, whisking often, until the butter is melted and the ingredients are combined.  Continue to cook, whisking frequently, until the mixture reaches a full boil that cannot be stirred down.  Boil 1-2 minutes, whisking vigorously.  (This will take quite a while - just be patient and keep whisking!)
7. Remove from heat, and beat the frosting with an electric mixer for 6-8 minutes, until lightened in color and thickened to a spreading consistency.  The frosting should be completely cool at this point.  Frost the cake immediately, before the frosting forms a crust.  The top of the frosting will become crackly and delicious as it cools.