Preheat oven to 350°F.

Grease or spray (3) 9” round baking pans. Line the bottom of each pan with parchment paper.

Using a medium bowl, whisk together cocoa powder and boiling water until smooth; set aside.

Using a large bowl, add flour, sugar, brown sugar, baking powder, baking soda and salt; whisk until well combined.

Add eggs, egg yolks, vegetable oil, sour cream and vanilla; whisk until well combined.

Finally, add the cocoa powder and hot water mixture to the bowl in several additions, whisking well between each addition.

Divide batter evenly into baking pans; tap pans lightly on countertop to release any air bubbles.

Bake at 350°F for 25-27 minutes, or until a toothpick inserted into center of cake comes out clean.

Let cakes cool in pan for 15 minutes and then transfer to a cooling rack until completely cool. (Note: Remove parchment paper from bottom of cakes when transferring them to the cooling rack.)

Using a countertop mixer, beat butter on medium speed until light and fluffy (~3 minutes).

Add powdered sugar 1 cup at a time, mixing on low speed after each addition.

Add cocoa powder, salt, vanilla extract and milk; mix on low speed until well combined.

Using a double boiler or microwave, melt chocolate. Pour melted chocolate into frosting. Beat on medium speed for 3-4 minutes, or until well combined. (Note: If you desire a looser frosting, add an additional tablespoon of milk. For a thicker frosting, add an additional ¼ cup of powdered sugar.)

Place one layer of cake on large platter. Spread ~¾ cup of frosting on top. Repeat with 2nd and 3rd layers of cake. Use remaining frosting on top and sides of cake.

{Optional} Garnish top of cake with chocolate curls.