Preheat oven to 180-degree Celsius. Line the cupcake moulds with liners.
In a bowl, add unsalted butter. Beat it with an electric beater or whisk until it becomes light and fluffy.
Then add castor sugar and beat it for a couple of minutes. Let it fully incorporate into the butter.
Pour vanilla essence and then add one egg into the mixture. Continue to beat and once it is combined well, add another egg. Beat again. This will prevent the batter from curdling.
Sift together all-purpose flour, cocoa powder and baking powder in a bowl. Whisk well. Then pour half of the dry-mixture into the egg mixture. Gently fold everything together.
Once it is fully combined, add the rest of the dry mixture. Continue to fold until well combined.
Pour the cupcake batter into the lined moulds and level by running a spoon over the top. Place it inside the preheated oven and bake for 25 minutes.
Take them out and insert a wooden skewer in the centre of the cupcakes, if it comes out clean, then place them on a wire rack to cool down. Otherwise, bake for a few more minutes.
After the cupcakes cool to room temperature, with the help of a knife scoop out a little portion of a cupcake. Fill it with a tablespoon of chocolate syrup and pipe the chocolate buttercream frosting over the cupcake. Repeat the same procedure for the rest of the cupcakes.
Soft and fluffy chocolate cupcakes filled with chocolate syrup are ready to serve.