Preheat the oven to 350°F (180°C). Line a cupcake tray with cupcake liners and set aside.
Sift the all purpose flour into a mixing bowl and add the sugar, baking soda and salt. Mix together.
Prepare the homemade buttermilk by adding 1 tablespoon of lemon juice to a measuring jug and then adding milk (room temperature) up to the ¾ cup (180ml) line. Let it sit for a minute or two to curdle.
Add the buttermilk, oil, egg and vanilla extract and mix into a batter.
Divide the batter evenly between the cupcake liners.
Bake for 20 minutes or until a toothpick inserted into one of the cupcakes comes out clean.
Transfer to a wire cooling rack to cool completely before frosting and when completely cool, frost with vanilla buttercream frosting.