For the cake, heat oven to 350F (176C). Spray three 9-inch (23cm) cake pans with cooking spray and line bottoms with parchment paper; set aside.
Sift together flour, cocoa, baking powder, baking soda and salt in a large bowl.
Beat sugar and oil together in a large bowl with a hand mixer for 1 minute. Add eggs one at time, beating well after each addition and continuing to beat for 2 minutes.
Stir together buttermilk, vanilla and espresso powder in a medium bowl.
Add 1/3 dry ingredients to egg mixture and beat together until almost combined. Add half the buttermilk mixture, stirring until mostly combined. Repeat additions ending with dry mixture, stirring until fully combined.
Divide batter evenly among pans and bake until a toothpick inserted into the centre of cakes comes out clean, about 45–55 minutes. Remove from oven and cool 10 minutes. Release from pans, remove parchment and place on a cooling rack to cool completely.
For the icing, beat butter in a large bowl with a hand mixer until smooth. Sift in cocoa and milk powder and beat to combine. Gradually add in 3 cups icing sugar, beating well after each addition.
Add whipping cream and beat to combine. Gradually mix in remaining icing sugar (3 cups). Beat in the vanilla and kosher salt.
Spoon a small dollop of icing onto cake stand. Place first cake round on top, pressing lightly to adhere.
Smooth ¾ cup (100g) icing onto cake round and top with second cake round. Repeat with more icing and another round. Use remaining icing to decorate top and sides of cake. Decorate with sprinkles if desired.