1 3/4 cups (210g) all-purpose flour
2 teaspoons pure vanilla extract
2 extra-large eggs, at room temperature
1/2 cup (112g) vegetable oil
1 cup (240g) buttermilk
1 teaspoon kosher salt
1 teaspoon baking powder
2 teaspoons baking soda
3/4 cup (90g) good unsweetened cocoa powder
2 cups (400g) sugar
1 cup (237g) freshly brewed hot coffee
5 tablespoons all-purpose flour
1 cup whole milk
1 teaspoon vanilla
1 cup (226g) butter
1 cup granulated sugar
16 ounces semi-sweet chocolate, chopped fine (can use milk chocolate)
16 ounces (2 cups) heavy cream, room temperature
Sift the flour, sugar, cocoa, baking soda, baking powder, and salt into the bowl and mix until combined.
In another bowl, combine the buttermilk, oil, eggs, and vanilla.
With the mixer on low speed, slowly add the wet ingredients to the dry. With mixer still on low, add the coffee and stir just to combine, scraping the bottom of the bowl with a rubber spatula. Pour the batter into the prepared pans and bake for 35 to 40 minutes at 350, until a cake tester comes out clean.
Cool in the pans for 30 minutes, then turn them out onto a cooling rack and cool completely
Pour cold milk into a small saucepan. Before turning on the heat, whisk in flour so no lumps remain.
Turn heat on to medium-low and heat until mixture is thick, like a roux, stirring the whole time.
Remove from heat, stir in vanilla and let cool completely.
Cream together butter and sugar on medium-high speed, until fluffy, about 8 minutes.
Add in the milk mixture and beat again until mixture resembles a whipped cream.
Heat the heavy cream in a quart-sized, microwavable container and microwave for 3 to 4 minutes on high, or until it just begins to simmer. Be careful not to allow the cream to boil over. Pour the cream over the chopped chocolate and let stand for 2 minutes.
Whisk the frosting until smooth.