Preheat oven to 350 degrees. Butter and lightly flour a 9-by-13-inch baking pan; set aside.
Combine chocolate and butter in a heatproof bowl set over simmering water; stir until melted and smooth. Set aside to cool slightly, about 5 minutes.
In a medium bowl, whisk together the flour, malted-milk powder, and malt balls; set aside. In the bowl of an electric mixer fitted with the whisk attachment, beat sugar and eggs until thickened and fluffy, about 2 minutes. Add melted chocolate mixture and vanilla; mix to combine, about 30 seconds more. Using a spatula, fold in flour mixture until just combined.
Pour batter into prepared pan, spreading it evenly with a rubber spatula. Bake until a cake tester inserted in the center comes out with just a few crumbs on it, 30 to 35 minutes. Transfer brownies to a wire rack to cool.
To crush malt balls, place them in a resealable plastic bag and push a rolling pin back and forth over top.