In a bowl sift the flour cornstarch, baking powder, and salt. Set aside.
In another bowl, add the butter, milk, and vanilla. Whisk until well-combined. Cover with a kitchen towel and set aside.
In two large bowls, separate 3 eggs into yolks and whites. Crack the last 2 whole eggs in the bowl with the egg yolks.
Pour 6 tablespoons of sugar into the egg yolk bowl. Mix on high until the batter is thick and pale yellow, about 5 minutes.
Beat the egg whites on high until frothy, then gradually add the 6 tablespoons of sugar. Continue beating until soft peaks form.
Gently fold in the egg yolk mixture into the egg whites.
Add the flour mixture, continue folding until the flour is mixed in. Create a well in the bowl, pour in the butter mixture. Fold all of the ingredients until the batter is smooth and silky.
Preheat oven to 350˚F (180˚C).
Pour half of the batter into a smaller bowl. Sift the cocoa powder over the batter. Fold in until the cocoa powder is fully incorporated.
In a greased parchment paper-lined 8-inch (20 cm) cake pan, spoon in a scoop of the vanilla batter in the center of the pan.
Spoon in a scoop of the chocolate batter over the vanilla in the center.
Repeat until you have poured about 6 to 8 layers, keeping in mind you will be spooning in a little less batter each layer.
Using a chopstick or skewer, gently draw a line from the edge of the pan to the center. Repeat until you have made 8 lines. Pour a little chocolate dot in the center.
Bake for 30 minutes, or until a toothpick comes out clean.
Use a knife or spatula to gently release the cake from the edge of the pan. Cover the cake with a greased plate. Flip the cake on to the plate then transfer to a wire rack to cool. Let cool for about 20 minutes before slicing.
For the strawberry sauce, heat a saucepan over medium heat. Add the strawberries and sugar, and stir until bubbling. Remove from heat.