Preheat your oven to 350°F (165C for Fan Oven). Grease and flour your cake tin.
Prepare your soured milk by adding 1 tablespoon of distilled vinegar into a 1 cup measure and filling the remainder with whole milk. Stir and let sit for 10 minutes while continuing to prepare batter.
In a large bowl, beat the sugar and butter together until light and fluffy. Add the eggs one at a time, beating well after each.
In a separate bowl, mix together dry ingredients - flour, baking powder, bicarb and salt.
Add your dry ingredients and soured milk in alternating additions to the sugar mixture. Be sure to scrape down the sides of your bowl along the way.
Add vanilla and beat until you have a smooth batter.
Pour into the prepared tin and bake for about 1 hour - until a skewer inserted in the middle comes out clean. Leave to cool in the tin for a few minutes before flipping out onto a wire rack to cool completely.