Steps for the 6" cake are labelled a; steps for the 8" cake are labelled b; steps for both sizes simply have numbers.

1a. Preheat oven to 350F/175C.  Place your oven rack in the centre of the oven.  Butter one 6"x3" round cake pan; line bottom with parchment.  Dust bottom and sides of pan with cocoa powder; tap out any excess.  

1b. Preheat oven to 350F/175C.  Arrange two racks in the centre of the oven.  Butter three 8"x2" round cake pans; line bottoms with parchment.  Dust bottoms and sides of pans with cocoa powder; tap out any excess.  

2. Sift cocoa into a medium bowl, and stir in boiling water. Set aside to cool.  

3. In a large bowl, sift together flour, baking soda, and salt; set aside.

4. In another large bowl, cream butter on low speed until light and fluffy.  Gradually beat in sugar until light and fluffy, 3 to 4 minutes, scraping down sides twice.  Beat in vanilla.  Drizzle in eggs, a little at a time, beating between each addition until the batter is no longer slick; scrape down the sides twice.  Set aside.

5. Mix milk into reserved cocoa mixture.  

6. With mixer on low speed, add 1/3 the flour mixture, then half the cocoa mixture, then 1/3 the flour mixture, then half the cocoa mixture, then the last 1/3 the flour mixture.

7a. Pour the batter into the prepared pan.  Place in hot oven.  Rotate the pan at about 30 minutes for even baking.  Bake until a cake tester inserted into the centre of the cake comes out clean, 45-60 minutes.

7b. Divide batter evenly among the 3 prepared pans.  Place in hot oven.  Rotate the pans at about 20 minutes for even baking.  Bake until a cake tester inserted into the centre of each layer comes out clean, 35-45 minutes.

8. Transfer pan(s) to wire rack(s); let cool 15 minutes.  Turn out the cake(s) and remove parchment; return to rack(s), top side up, until completely cool.

9a. Cut the cake in half vertically, giving yourself two layers.

10a. Reserve the prettiest layer for the top.  Place one cake layer on a serving platter; spread with frosting.  Add the second cake layer; spread with more frosting on top and down sides of cake.  Serve.

10b. Reserve the prettiest layer for the top.  Place one cake layer on a serving platter; spread with frosting.  Add the second cake layer; spread with more frosting on top of layer.  Top with third cake layer; cover outside of cake with the remaining frosting.  Serve.