Adjust oven rack to middle position and preheat oven to 350°F. Spray two 8-inch round cake pans with baking spray. Set aside.

In the bowl of a stand mixer fitted with the paddle attachment, stir the sugar, flour, cocoa, baking powder, baking soda, and salt together on low speed for 30 seconds.

Add the eggs, buttermilk, oil and vanilla; increase speed to medium and beat for 2 minutes.

Reduce speed to the lowest speed and slowly add the boiling water. Stir on low for 1 minute.

Pour the batter evenly between the prepared cake pans and bake for 25-35 minutes or until a toothpick inserted in the middle comes out clean. Cool 10 minutes in the pan on a cooling rack, then remove from the pans. Return the cakes to the cooling rack to completely cool.

For the Chocolate Frosting: In a medium saucepan set over medium heat, whisk the sugar, flour, cornstarch, salt, and cocoa together.

Slowly add the milk, while whisking briskly, and beat until smooth. Cook, whisking constantly until mixture boils.

Reduce heat to medium-low, and continue cooking and whisking until mixture is very thick, about 4-8 minutes. Transfer the mixture to a wide bowl, and place plastic-wrap directly on the surface of the chocolate/milk mixture. Set aside to cool completely, about 2 hours.

Once the chocolate/milk mixture has cooled, fit a stand mixer with the paddle attachment and add the butter to mixing bowl. Beat the butter on medium-high speed until lighter in color and fluffy, about 5 minutes.

Reduce speed to medium speed and add the cooled chocolate/milk mixture and vanilla. Beat together until incorporated.

Increase the speed to medium-high and beat until frosting is light and fluffy, about 3-5 minutes. Add the melted (and cooled) chocolate, and beat until melted chocolate is incorporated.

Frost the cake with the completed chocolate frosting, cut into slices, and serve.