Preheat the oven to 350 F and line your baking tray with cupcake liners and keep aside.
In a large mixing bowl, combine flour, baking powder, and salt and mix until well incorporated.
In another bowl add oil, sugar, and vanilla extract and mix well.
Now, add the egg and milk to the wet mixture and mix until combined.
Add the flour mixture to the wet mixture in batches and beat using electric hand mixer on low until just combined.
Scoop out the batter using an ice cream scoop, and fill your liners till 3/4 full.
Bake in a preheated oven for 22-25 minutes or till a toothpick inserted in the middle comes out clean. Let the cupcakes cool for 5 minutes in the baking tray before transferring them to a wire rack to cool completely.
Once the cupcakes are completely cooled, using the back of a piping nozzle, make a cut at the centre to create a well and remove that part (don't throw it away).
Fill it with jam generously, and close it using the cut-out part.
Make the Frosting
In a bowl of a stand mixer or using a hand mixer, cream the butter until it's light and creamy. Add powdered sugar (in batches) and beat until you get a smooth and fluffy consistency (5-6 min).
Fill a piping bag with the prepared frosting and attach a star-tip (or any other) nozzle. Pipe the swirls of frosting on top of the cupcake along the edges, keeping the center empty.
Add a little bit of jam in the centre again using a piping bag or small spoon. Sprinkle some gold sugar crystals on top if desired.