Make the frosting: Sprinkle the sugar evenly in a large skillet and add the corn syrup. Bring to a boil over medium heat, stirring once or twice. Continue to boil, swirling the pan but not stirring, until the caramel is amber. Carefully pour in the heavy cream (it can splatter) and add the butter. Simmer, whisking constantly, until the caramel is smooth, about 2 minutes.
Combine both chocolates and the salt in a medium heatproof bowl. Pour the hot caramel over the chocolate and let stand until melted, about 5 minutes. Whisk until smooth and shiny, then cover and refrigerate until set, at least 4 hours or overnight. (The frosting can be made up to 2 days ahead.)
Make the cupcakes: Position a rack in the middle of the oven and preheat to 375 degrees F. Line two 12-cup muffin pans with paper liners. Whisk the flour, baking powder and salt in a bowl. Sift the malted milk and cocoa powders into another bowl.
Beat the sugar and butter in a third bowl with a mixer until light and fluffy, about 7 minutes. Add the malted milk powder mixture and beat until combined. Add the eggs, one at a time, beating well after each addition. With the mixer on low speed, add the flour mixture in three parts, alternating with the half-and-half in two parts, beginning and ending with flour.
Divide the batter evenly among the prepared cups, filling each about three-quarters of the way. Bake until a toothpick inserted into a cupcake comes out clean, 15 to 20 minutes. Let cool in the pans 10 minutes, then transfer the cupcakes to racks to cool completely.
Beat the chilled frosting with a mixer or whisk until thick, about 2 minutes, but don't overwhip. Spread on the cupcakes or transfer to a pastry bag with a star tip and pipe on top. Refrigerate until ready to serve, then sprinkle with the cereal.