For the Marble Cake:
Preheat oven to 350 degrees . In a bowl, whisk the flour, baking powder and salt.
In an electric mixer fitted with the whisk attachment, beat the butter on high until creamy, 2 minutes. With the machine on, gradually add the sugar and beat, scraping the bowl, until fluffy, 8 minutes. Beat in the eggs one at a time, scraping the bowl. Beat in the vanilla. At low speed, add the flour mixture in three additions, alternating with the milk. At medium speed, mix until smooth.
Transfer 2 cups of batter to another bowl; stir in cocoa powder. Divide and pour the remaining yellow batter into two foil-lined 9-inch round cake pans. Spoon blobs of cocoa batter on top of the yellow batter. Using a butter knife, swirl in the cocoa blobs. Bake for 35 minutes; invert and let cool on a rack.
For the Chocolate Malt Frosting:
In an electric mixer, beat the butter and salt at medium speed until creamy about 1 minute. Beat in melted and cooled chocolate and vanilla until thoroughly combined, about 1 minute.
Dissolve malt powder into the cream. Set aside.
Sift in the confectioners' sugar, 1 cup at a time, mixing at low speed to incorporate (the mixture will be crumbly). Beat at medium speed for 1 minute, then increase to high speed and beat until smooth, 3 to 4 minutes. Gradually add cream and malt mixture then beat until fluffy, 2 to 3 minutes.
Place one cake, rounded side up, on a cake stand. Spread with 1 cup Chocolate Malt Frosting. Sprinkle with 1/3 cup chopped malted milk balls. Top with the remaining cake, flat side up. Spread with the remaining frosting. Decorate with more candies.