Make the strawberries by combining them with the vinegar and caster sugar in a shallow rectangular dish (about 8x5 inches). Stir everything together, spread evenly (some berries overlapping are ok), and set aside for 10 minutes.
Preheat the oven to 200 C, then bake the strawberries for 20 to 25 minutes until softened but not completely broken down, stirring a couple of times to ensure they cook evenly. (You can cook them longer for a more jammy or sauce-like texture too; I prefer to leave the berries a little intact). Set aside to cool completely at room temperature and the liquid will thicken a little.
Make the cake. Keep the oven preheated at 175 C. Grease a 9 inch springform pan and set aside.
Beat the butter and sugar in a large mixing bowl with a hand mixer on medium-high speed till pale and fluffy. Add the eggs and vanilla and beat well till combined.
Sift in the flour, baking powder and salt. Fold gently till combined and then add the milk. Beat briefly with the hand mixer on low to make a smooth, thick batter.
Spoon the batter into the prepared tin and smoothen it out.
Bake for 30 mins, then check to see if the top is browning too fast. If so, cover with aluminium foil and continue to bake for another 15 to 20 minutes until done. A toothpick poked in the center should come out clean.
Let the cake cool completely at room temperature before frosting.
Make the frosting. Beat the mascarpone, icing sugar and vanilla till smooth with a hand mixer on low speed. Add the cream and beat again on medium-high speed till light, fluffy and thickened, 2 to 3 minutes. When you lift the beaters, the mixture should hold soft peaks. Keep chilled while you slice the cake.
Unmould the cake, and using a sharp, long knife, level the top, then slice it in half horizontally, turning it with your hands as you go, so that the cut is even (or use a cake turntable if you have one). Use a metal spatula to lift the top off and set it aside on a large tray or plate. Transfer the lower half to a serving platter.
Spoon about half of the frosting onto the lower half, spreading it evenly, then top with two-thirds of the cooked strawberries, taking care not to spoon on too much of the liquid left behind in the dish.
Place the top half of the cake on the berries, then spread the rest of the frosting over it, letting it spill over the sides. Use a blunt knife or metal spatula to spread it evenly all over the cake. Pile on the remaining strawberries. Chill the cake for about 30 minutes minutes before slicing so the frosting holds its shape.
Optional: If the liquid left behind in the baking dish (after scooping out the strawberries) hasn't thickened to your liking, you can cook it down further to a sauce-like consistency. Pour it into a small saucepan and then mix 1 tsp water with 1/4 tsp cornflour in a small bowl, and add it to the pan. Bring to a boil for a couple of minutes and set aside to cool. Spoon this sauce over individual pieces of cake.
The cake must be kept refrigerated at all times (it will keep for 4 to 5 days) but leave it out at room temperature for 10 to 15 mins before digging in because butter cakes harden in the fridge. Happy baking!