In a bowl add milk and add vinegar to it. Mix it. Keep it aside for 5 minutes. The milk will start curdling as shown.
In the meantime, sift the dry ingredients (all purpose flour, custard powder, baking soda and salt). Line the muffin mould/tray with cupcake liners and keep aside.
In a mixing bowl, add oil and sugar. Beat them with a hand whisk till sugar dissolves.
Now add the curdled milk to it and mix them well for 2 minutes.
Add 2 tbsp of plum jam and mix well.
Mix them well.
Add the dry ingredient gradually in batches and mix them gently.
The cake batter is ready. Do not overmix them.
Fill the batter with the help of spoon in the cupcake liners. It will be around two tablespoon per mould.
Put the remaining 2 tbsp plum jam on the batter as shown in the pic.
Tap the cupcake tray gently and bake in the preheated oven @ 180 C for 18-20 minutes. The time varies from oven to oven.
After 18-20 minutes check and remove the mold. Remove from the oven and cool them. The cupcakes are ready to use now.
The cupcakes are ready to serve. Serve them warm with cup of tea or coffee. They can be refrigerated for a week in airtight container.