Preheat oven to 170°C. Grease two 22cm round (base measurement) cake pans. Line bases and sides with baking paper. Sift the flour, ginger, cornflour, baking powder, bicarbonate of soda and salt in a large bowl. Stir in the sugar.
Combine the milk, sour cream and cordial in a separate bowl. Heat in the microwave on high for 30 secs. Stir well. Gradually whisk into the flour mixture with butter, stirring until just combined.
Use an electric mixer to whisk egg whites in a clean, dry bowl until light and fluffy. Fold into the butter mixture, in 2 batches, until just combined. Divide among prepared pans. Bake for 50-55 mins or until a skewer inserted in centre of cakes comes out clean. Cool in the pans for 10 mins before turning onto a wire rack to cool completely.
Meanwhile, to make the Swiss meringue buttercream, whisk egg whites and sugar in a heatproof bowl until combined. Place bowl over a saucepan of simmering water. Whisk until mixture reaches 71°C on a sugar thermometer.
Use an electric mixer to whisk the egg white mixture until stiff peaks form. Stand for 5 mins or until meringue cools to room temperature. With the mixer on medium speed, add the butter, 1 piece at a time, beating well after each addition. Beat in the vanilla and salt.
Use a serrated knife to trim the top of each cake to level. Place a dollop of the buttercream on a plate. Top with 1 cake, cut-side up. Spread with one-third of buttercream and sprinkle with half the crushed biscuits. Place the remaining cake, cut-side down, on top. Use an offset spatula to evenly smooth the buttercream over the top and sides of the cake. Decorate with candy canes and remaining biscuit.