Preheat oven to 350 degrees F. Line standard muffin tins with paper liners. Whisk together flour, cocoa, sugars, baking soda, and salt. In another bowl, whisk together the milk and malted milk powder until the powder is dissolved.
With an electric mixer on medium-high speed, beat flour mixture, milk mixture, and oil until combined. Add eggs, one at a time, beating until each is incorporated, scraping down sides of bowl as needed. Add sour cream and vanilla, and beat until just combined.
Divide batter evenly among lined cups, filling each halfway. Bake, rotating tins halfway through, until a cake tested inserted comes out clean, about 20 minutes. Cupcakes can be stores up to 3 days at room temperature, or frozen up to 2 months, in airtight containers.
For the buttercream:
Beat butter, vanilla, and salt with an electric mixer until smooth. In a separate bowl, sift together sugar, cocoa powder, and malted milk powder. Gradually add sugar mixture alternatively with the milk in 3 additions. Beat until smooth, adding more milk until desired consistency is achieved. Remove frosting to a piping bag fitted with a tip of your choosing and pipe onto cupcakes. Place a malt ball on the top of each cupcake. Decorate with a striped paper straw cut to size, if desired.