Diablo Cake (not vegan)

This was the first layered cake I've ever made. And the first decorated. <3 I made it for Easter and took it over to my aunt's Easter-party thing. They all said it was good, so I'm counting on their feedback:D It took foreverrrrr, so I'm just going to post the standard 8-inch round layered cake.

Serves: I'm not sure. However many people it takes to eat a whole cake.
Diablo Cake:
1 1/2 cups all-purpose flour
1 teaspoon baking powder
2 teaspoons baking soda
1/2 teaspoon salt
1 cup hot brewed coffee
1 cup unsweetened cocoa powder, plus extra for dusting
2 sticks unsalted butter
1 cup light brown sugar
1 cup sugar
3 eggs
1 cup buttermilk
1 tablespoon vanilla extract

Preheat oven to 350 degrees. Grease two 8-inch round cake pans and dust them with cocoa powder.
In a large bowl, sift together the flour, baking soda, and salt. Set aside. In a small bowl, whisk the hot coffee and cocoa powder together. set aside.
Using a standing mixer, cream the butter on low speed for about one minute. Add the sugars and beat on low speed for 3 minutes until light and fluffy. With the mixewr running, add the eggs one at a time. Then add the buttermilk and vanilla extract until thoroughly combined. Alternate adding the dry ingredients and the chocolate-coffee mixture, starting and ending with the dry ingredients. When the mixture is almost combined, turn off the mi9xture and finish mixing by hand (to avoid overbeating). Pour the batter into the prepared pans and bake for about 35 to 40 minutes, or until your tester comes out clean. Cool the pans for 10 minutes, then invert onto a wire rack.

Vanilla Whipped Buttercream:
2 sticks (1 cup) unsalted butter
1 cup sugar
1 cup whole milk
1/4 cup all-purpose flour
1 1/2 tablespoons vanilla extract

Cream the butter on medium speed, 3 to 5 minutes, in a standing mixer until soft. Add the sugar and beat on high until light and fluffy, 5 to 7 minutes.
In a small saucepan, combine 1/4 cup of the milk, the flour, and the vanilla and whisk until there are no lumps. Over medium heat, slowly add the remaining 3/4 cup milk, whisking constantly, and cook until the mixture comes to a low boil. Then, reduce heat to low and keep whisking for a few minutes until the mixture starts to thicken.
Immediately remove the saucepan from the heat, but keep stirring. If necessary, place the pan over a bowl of ice water to stop the cooking process and allow the mixture to cool.
Once the mixture has thickened, stick it in the freezer to cool (take it out when it is room temperature. Make sure it isn't warm otherwise it won't set.)
With the mixture on low speed, slowly pour the milk mixture into the butter-sugar mixture. Increase the speed to medium and beat until the frosting is light and fluffy, 3 to 5 minutes. Add whatever food coloring you want.
Frost your cake. [:

*For an amazing filling, dissolve 1 heaping teaspoon of instant espresso in 1 tablespoon boiling water and combine with the butter-sugar mixture. It works beautifully with this cake.*

**Make sure to really cream the butter and sugar together, don't hold back.**