Preheat oven to 350°F. Grease and flour 3 8-inch cake pans.
In a standing mixer or mixing bowl with handheld electric mixer, cream shortening and sugar on medium speed. Add eggs and beat until combined.
Combine cocoa powder, food color, 3 tablespoons of buttermilk in a small bowl. Stir to make a paste and add to the mixer on low speed.
Add half the buttermilk, then half the flour to the mixing bowl on low speed. Add salt, then remaining buttercream, then remaining flour.
Increase speed to medium-low. Add vanilla and mix to incorporate, scraping down sides if needed.
In a small bowl, add baking soda to vinegar then add to the mixer and combine.
Divide the batter evenly between the three pans. Bake for 25 to 30 minutes. (If using the toothpick method to check doneness, it’s ok if there are a few crumbs clinging to the toothpick).
Let cool while preparing the buttercream.
In the bowl of a standing mixer, beat the butter on medium speed until soft and pale yellow. Turning the speed to low, add confectioners sugar 1 cup at a time until incorporated.
Add vanilla extract and increase speed of the mixer to medium. One tablespoon at a time, add the heavy cream until a smooth and creamy consistency is achieved.
Transfer 1 ½ to 2 cups of the white buttercream to a piping bag for the crumb coat. Reserve an additional ½ cup frosting for each of the blues.
With the remaining buttercream, add the black food gel color a little at a time until a dark grey/black color is achieved. Transfer the frosting to a large piping bag fitted with a large round tip.
Assembling the White Walker Dragon Cake
Once the cakes are cooled, pipe the white buttercream between each of the layers and smooth over the top and sides. It's ok if some crumbs stick to the cake. Transfer the crumb coated cake to the refrigerator until frosting is stiff and set.
While crumb coating is setting, prepare the dyed frostings. In each bowl with the ½ cups of reserved frosting, add a little gel food color at a time and mix until desired hue is achieved.
Once crumb coat is set, pipe large round dots in an uneven column down the side of the cake. Using the icing spatula, lightly scoop a very tiny amount of one or the other blue buttercream - or a little of both - frostings.
Press down on ¾ of one round piped dot with the tip of the icing spatula on the large round circle and drag it across to give the thumbprint pattern with the streak of blue (see note.) Repeat for the remaining dots in the 'column' then start a new one.
Once the sides are complete, using the same method work in an uneven circle starting at the center of the cake and working to the edge.
Dust the sides and top of the cake with silver shimmer dust. Store in the refrigerator, serve at room temperature.
Food coloring: if using liquid food color for the cake (thinking baking aisle at the grocery store), use 2 fluid ounces. The gel food coloring can be found at any craft store in the baking department.
After a few smears with the icing spatula you may need to wipe it off on the cutting board or the side of a bowl so the black icing doesn't combine with the blue.