Preheat oven to 350°F/180°C. Grease and flour 3 (9-inch) cake pans.
Sift together flour, sugar, cocoa, baking soda, baking powder, and salt.
Mix together milk and malted milk powder in a small bowl to dissolve.
Add sifted ingredients, milk mixture, oil, and eggs into bowl of the stand mixer. Beat briefly at medium speed until smooth.
Gently mix in sour cream and vanilla until combined.
Pour batter evenly into prepared pans, and bake for 20 minutes, or until a toothpick inserted in center comes out clean.
Let cool in pans for 10 minutes. Remove cakes from pans and let cool completely on wire racks.
For the Malted buttercream
Beat butter in stand mixer until light and fluffy. Gradually add in the malted milk powder and then the sifted powdered sugar and vanilla. Add milk to achieve the desired consistency.
Use the buttercream to fill and top the cake. Decorate as desired.