In a bowl, mix the crushed cornflakes with melted butter until fully combined. Press the mixture firmly into the bottom of a 9-inch (22 cm) springform pan or cake ring. Smooth with the back of a spoon and freeze for at least 15 minutes while preparing the other layers.

Melt the white chocolate in the microwave in 10-second bursts, stirring between each until fully melted. Stir in the whole cornflakes, ensuring they are well coated. Set aside at room temperature until ready to assemble.

Melt the white chocolate and oil together until smooth. Stir in the crushed cornflakes until evenly coated. Spread the mixture in a thin layer on a parchment-lined baking sheet. Freeze for 10 minutes, then break into small crunchy pieces.

In a large mixing bowl or stand mixer fitted with a whisk, combine mascarpone, cold heavy cream, and strained cornflake-infused milk. Add sugar and whip on medium speed until the mixture thickens and holds soft peaks. Add the vanilla pudding mix and whip again until fully incorporated and stable. Gently fold in half of the crunchy topping (reserving the rest for garnish).

Remove the chilled base from the freezer. Pour half of the mascarpone cream over the base and smooth the top. Evenly distribute the chocolate-coated cornflakes layer on top (reserve a few for garnish). Pour the remaining mascarpone cream over the cornflakes layer and smooth the top. Cover and freeze for at least 5 hours or until fully set.

In a small saucepan, heat the heavy cream over low heat until warm but not boiling. Pour over the chopped white chocolate and let sit for 1 minute, then stir until smooth. Let cool slightly before pouring over the frozen cake.

Drizzle the white chocolate ganache over the frozen cake. Sprinkle with reserved crunchy topping and a few whole corn flakes for decoration. Keep frozen until ready to serve. Remove from the pan just before serving and slice with a warm knife.