Make the Cake: Preheat oven to 350 degrees F. Grease two 8-inch round cake pans, line the bottoms with parchment, grease the parchment, and dust with flour.
In a medium bowl, sift together the flour, malted milk powder, cocoa powder, baking soda and baking powder; set aside.
Beat the butter and sugar with an electric mixer on medium speed until light and fluffy, about 3 minutes. Add the eggs, one at a time, beating well after each addition (after the second and third eggs the batter may look curdled, but once you give it a good mix, it will smooth out). Add the melted dark chocolate and the Lyle's golden syrup and mix until well combined.
In a large (2-cup) measuring cup, whisk together the milk, sour cream and vanilla extract.
Add one-third of the flour mixture to the batter and mix on low speed until combined, then add half of the milk mixture, beating well. Repeat with another third of the flour mixture and the remaining milk mixture. Finish with the final third of the flour mixture. Using a rubber spatula, fold the crushed Whoppers into the batter, ensuring that no dry streaks remain.
Divide the batter between the cake pans and smooth the top into an even layer. Bake until a skewer inserted into the center comes out with just a few moist crumbs attached, 45 to 55 minutes. All the cakes to cool in the pans for 5 minutes, then turn them out onto a wire rack to cool completely.
Make the Frosting: Place the white chocolate and heavy cream in a medium heat-proof bowl set over a small saucepan of barely simmering water. Stir occasionally until completely melted. Remove from the heat and set aside to cool to room temperature.
8. Beat the butter with an electric mixer on medium-high speed until smooth and creamy, about 4 minutes. Reduce the speed to medium-low and gradually add the powdered sugar. Once all of the sugar has been incorporated, increase the speed to medium-high and beat until light and fluffy, about 3 minutes. Reduce the mixer speed to medium, add the malted milk powder and vanilla extract and beat until well combined. Gradually pour in the cooled melted white chocolate mixture, beating until smooth.
Assemble the Cake: Using a cake divider or long serrated knife, slice each cake layer in half horizontally so you have four layers. Place one layer on a serving dish and spread with ¾ cup of the frosting. Top with a second layer of cake, then spread again with another ¾ cup of the frosting. Repeat with the final two layers of the cake. Using a large offset spatula, cover the outside of the cake with the remaining frosting. Decorate the top with the speckled malted eggs.
Lyle's Golden Syrup is a sugar syrup that falls between honey and molasses in terms of color and flavor. I purchase it in the British section of the international foods aisle in my local grocery store. If you can't find it locally, it can also be ordered from King Arthur Flour or Amazon.
The frosting can be made the day before, covered and refrigerated. Remove from the refrigerator 20 minutes before frosting the cake, to allow it time to soften.
The baked cake layers can be frozen, wrapped in plastic wrap, for up to 3 months. Defrost fully before frosting the cake.
I assembled this entire cake the day before I was going to be serving it and since the frosting was on the softer side, I refrigerated the cake. I thought that doing this caused the cake layers to become too dense (they were softer and more sponge-like when initially assembling the cake), so I would not recommend refrigerating before serving. It ended up sitting out all day once it was served and the frosting never drooped or slid, so I don't think it would be an issue.