Preheat the oven to 180°C and line a 12-cup muffin tin with large paper liners.
Using electric beaters or a stand mixer fit with the paddle attachment, beat the butter and sugar together on high speed for 1 minute. Add the eggs one at a time, beating well and scraping the bowl after each addition, then beat in the vanilla.
In a separate bowl, sift the flour, cornstarch, baking powder and salt. Add this alternately with the sour cream in 3 additions, starting and ending with the flour. Spoon this into the prepared muffin tin.
Bake the cupcakes for about 18 minutes, until a tester inserted in the centre of the cake comes out clean. Let the cupcakes cool for 15 minutes before removing to cool completely before frosting.
To prepare the frosting, whip the cream until it holds a soft peak then set aside. Beat the cream cheese and butter together until light and fluffy. Beat in the icing sugar and vanilla, and then fold in the whipped cream in 2 additions. Using a piping bag fit with a large star tip, pipe the frosting onto each cupcake. Insert the strawberry slices into the cream, arranged to look like flower petals – once the frosting chills, it holds the slices securely. Chill until ready to serve.
The cupcakes are best eaten the day they are assembled (you can make the cupcakes a day ahead and store at room temperature).