Preheat the oven to 160℃/320℉ and line a muffin tin with liners.

Separate egg whites and yolks. Set aside the whites.

Into the egg yolks add the vegetable oil, milk, and vanilla extract. Whisk till combined. Sift in all-purpose flour, cornflour/cornstarch, and salt. Combine with a spatula. Set aside.

To the egg whites add cream of tartar and beat with the help of a hand or stand mixer till frothy. Start adding sugar little by little. After the addition of sugar beat on medium-high speed till soft peak appears.

Add 1/3rd part of the egg whites into the yolk mixture. Combine gently with a spatula. Add this mixture to the remaining egg whites and fold in gently with a spatula till combined.

Pour the batter into the prepared muffin tin till 90% full. Bake in the preheated oven at 160℃/320℉ for 10 minutes then reduce the temperature to 110℃/230℉ and bake for 40 minutes.

Transfer the cupcakes to a wire rack and cool completely.

Pour the milk and vanilla into a medium-sized saucepan and bring it to a boil. Set aside to cool down slightly.

To a medium/small-sized saucepan add the egg yolks and sugar. Whisk till properly combined. Add the cornflour/cornstarch and whisk to combine.

Add the warm milk slowly to the mixture while whisking continuously. Cook the mixture on medium-low heat while stirring continuously. After the mixture has thickened cook for another 1 minute or so.

Remove from heat and add cold butter. Mix till butter is combined. Transfer the custard to a bowl and cover the surface with a cling wrap. Refrigerate for a few hours.

Beat heavy cream with a mixer for a few minutes. Add powdered sugar and beat till the cream has thickened. Add the chilled custard to the cream and beat till evenly combined. Fill the custard cream in a piping bag fitted with any nozzle.

Create slits in the center of the cupcakes with the help of a small knife.

Insert the nozzle into the slit and fill the with custard cream till it overfills.