Make the cupcakes: Preheat the oven to 350˚. Line a 12-cup muffin pan with paper or foil liners. Whisk the flour, malted milk powder and baking soda in a medium bowl.
Heat the milk until hot but not boiling; pour over the cocoa powder in a large bowl and whisk until smooth. Let cool slightly.
Whisk the granulated sugar, vegetable oil, egg and vanilla into the cocoa mixture until smooth. Whisk in the flour mixture until just combined.
Divide the batter among the prepared muffin cups. Bake until the tops spring back when gently pressed, 20 to 25 minutes. Transfer to a rack and let cool 5 minutes in the pan, then remove the cupcakes to the rack to cool completely.
Meanwhile, make the frosting: Beat the butter, cream cheese and vanilla in a large bowl with a mixer on medium speed until creamy, 1 to 2 minutes. Gradually beat in the confectioners' sugar on medium-low speed until smooth, then increase the speed to medium high and beat until thick and fluffy, 1 to 2 more minutes.
Spread the frosting on the cupcakes and sprinkle with white nonpareils. Stick a candy-melt tree into the center of each cupcake.