Preheat oven to 180C / 355F. Line a muffin tin with 4 cupcake liners.
Place butter and sugar in a mixing bowl. Sift in flour, baking powder and salt. Add in egg and vanilla. Using a handheld electric mixer, beat on low for 30 seconds, then turn the speed up to medium and beat for 1 minute. Scrap down the sides of the bowl with a rubber spatula and beat on medium speed for another 1 minute.
Fill the paper liners until ⅔ full (¼ cup batter per standard cupcake). Bake for 18 to 20 minutes until a skewer inserted into the center of the cake comes out clean.
Remove the cupcakes and let cool completely before frosting.
Vegan Chocolate Frosting
Sift cornstarch, cocoa powder and salt into a small pot. Add in sugar and water and whisk with a wire whisk until there are no more lumps.
Place the pot over medium-high heat and whisk constantly with the wire whisk. When the mixture starts to thicken, remove from heat and whisk vigorously until the frosting is homogeneous. Return to medium heat and boil for 30 seconds to 1 minute, whisking constantly until the frosting is thick and spreadable. Remove from heat and whisk in oil and vanilla. Let cool completely before using.
Spoon a heaping spoonful of Vegan Chocolate Frosting on top of each Vanilla Cupcake and frost with a palette knife or the back of a spoon. Leftovers can be kept in an airtight container in the refrigerator for up to 5 days.