Separate the egg yolks and egg whites into 2 different bowls. Set aside.
In a large bowl, sift together the dry ingredients: all purpose flour, cornstarch, baking powder and lemon zest. Set aside.
Preheat oven to 350°F/177°C. Line a 8" by 4" loaf pan with parchment paper.
In a clean stand mixer bowl fitted with a whisk attachment, transfer the egg whites into it and whip until foamy. Add in the cream of tartar and keep beating until lots of bubbles form.
Gradually add in 50g of granulated sugar and whip until stiff peaks. (Peak will be straight when whisk is lifted).
Remove the whipped egg whites and transfer to a large clean bowl. Set aside.
Using the same mixer bowl, change to a paddle attachment and add in the butter and remaining 100g granulated sugar.
Cream until light and fluffy, about 2-3 minutes on high speed.
Gradually add in the egg yolks one by one, and then slowly add the milk, lemon juice and vanilla extract. Beat until combined. (At this point, it may begin to curdle, which is fine).
Remove the mixer bowl and sift about ⅓rd of the dry ingredients (flour, cornstarch, and baking powder) over the egg yolk batter.
With a spatula, fold in the dry ingredients into the mixture and alternate with ⅓rd of the whipped egg whites, repeating until everything is combined together and there are no white streaks left, but making sure not to overmix as to deflate the batter.
Remove about ⅓rd of the cake batter and fold it into the raspberry coulis (from earlier) to make the raspberry layer.
Transfer about ⅔rds of the remaining plain cake batter into the lined loaf pan.
Add the raspberry layer and cover with the remaining plain cake batter.
Use a sharp knife to swirl the two batters together.
Carefully tap the pan a few times to release any trapped air bubbles.
Bake at 350°F/177°C for 60-65 mins, until golden brown on top, or until a toothpick inserted comes out clean, or with the slightest crumbs clinging to it.
Let cool completely before icing.