Combine milk and lemon juice and stir well. Set aside for 15 minutes to make buttermilk.
Preheat oven to 180 degrees Centigrade.
Grease a square baking tin lightly with butter.
In a mixing bowl, using a stand or hand held mixer, beat butter and sugar on low speed until light and fluffy, about 8 minutes.
Scrape down the sides of the bowl while creaming.
Add eggs one at a time, beating on low until incorporated. Continue to beat on low until batter is creamy.
In a separate bowl, whisk together the flour, baking powder, and salt.
Add 1/3 of the dry ingredients to the batter. On a low speed, mix the ingredients just until you see no flour streaks.
Add 1/2 the buttermilk and mix on low again. Repeat flour and buttermilk addition alternately. You should end with the flour mixture.
When all ingredients have been mixed, add the vanilla and mix on low until just incorporated.
Pour batter into the prepared pan and evenly smooth the top with a spatula. Bake at 20-25 minutes or until a toothpick inserted into the cake comes out clean with no crumbs.
Let cake cool completely after baking.