Preheat the oven to 350°F (180°C) and prepare an 8-inch (20cm) loaf pan by lightly greasing the bottom of the pan and the sides of the baking sheet with oil and line it with an overhang of baking paper.
In a medium sized mixing bowl combine milk, vinegar and yogurt. Allow this mixture to sit for a few minutes until it is slightly curdled.
Add sugar, oil and vanilla extract, mix using a whisk until the sugar is dissolved.
In a separate bowl combine all the dry ingredients: all-purpose flour, cornstarch, baking powder, baking soda salt.
Sift dry ingredients into the liquid ingredients and mix.
Pour 3/4 of the prepared batter in a separate bowl. To this add sifted cocoa powder and red food color. Mix to get a smooth red colored batter.
Pour half of the red cake batter into the base of the loaf pan followed by all the vanilla batter and top it with the remaining red cake batter.
Use an offset spatula to smooth the top and then create swirls using a knife.
Bake the loaf cake for 40-45 minutes, or until the top of the cake is firm to touch and a skewer inserted into the center of the cake comes out with a few crumbs attached.
Cool the cake in the loaf pan on a wire rack completely for 45 minutes. Then remove it from the loaf pan and allow it to cool completely on the wire rack.