2) Butter the bottom of two 20cm cake pans. Then line them with baking paper and butter, flour the lined pans.
3) Cream the butter and sugar on high speed in the bowl of an electric mixer fitted with the paddle attachment until light and fluffy. On medium speed, add the eggs, 1 at a time, then the sour cream, zests, and vanilla, scraping down the bowl as needed. Mix well. Sift together the flour, cornstarch, salt, and baking soda. On low speed, slowly add the flour mixture to the butter mixture and combine just until smooth.
4) Pour the batter evenly into the pans, smooth the tops, and bake in the centre of the oven for 40 to 45 minutes, until a toothpick comes out clean. Let cool in the pans for 30 minutes, then remove to wire racks and let cool to room temperature. If using 1 cake, wrap the second well and freeze.
For the filling: 1) Whip the cream, sugar, and vanilla in a mixer fitted with the whisk attachment until firm.
2) Slice one of the cakes in half with a long, sharp knife. Place the bottom slice of the cake on a serving platter, spread with 1/2 the whipped cream and scatter with sliced strawberries. Cover with the top slice of the cake and spread with the remaining cream.