Chocolate Malt Cupcakes

For the cupcakes:

• 2 1/4 cups all-purpose flour
• 3/4 cup unsweetened Dutch-process cocoa powder
• 1/2 cup granulated sugar
• 3/4 cup packed light-brown sugar
• 1 1/2 teaspoons baking soda
• 1/2 teaspoon salt
• 1 cup milk
• 1 1/4 cups malted milk powder
• 1 cup vegetable oil
• 3 large eggs, at room temperature
• 1 cup sour cream, at room temperature
• 1 teaspoon pure vanilla extract

For the buttercream:

• 1 cup (2 sticks) unsalted butter, at room temperature
• 2 teaspoons pure vanilla extract
• 1/4 teaspoon salt
• 5 1/2 cups powdered sugar
• 1 cup unsweetened cocoa powder
• 1 cup malted milk powder
• 1/2 cup milk, plus more if necessary
• chocolate covered malt balls to garnish
• red and white striped paper straws to decorate (optional)