Preheat oven to 350 F degrees. Line a 12 count muffin pan with cupcake liners or spray with nonstick spray and set aside.
In a bowl sift together all purpose flour, salt, baking soda and baking powder. Set aside.
In the steel bowl of your stand mixer fitted with paddle attachment or using your hand mixer, beat together melted butter and granulated sugar till well combined.
Add in the eggs and vanilla extract and beat for 3-4 minutes till mixture if smooth and well combined.
Start adding flour mix to the wet ingredients alternating with buttermilk starting and ending with flour.
Mix till everything is well combined but do not overmix.
Fill the cupcake liners with the batter till 2/3rd full.
Bake at 350 F degrees for 17-20 minutes or till a toothpick inserted in the center comes out clean.
Lemon curd
While the cupcakes are baking, start making the lemon curd.
In a bowl whisk together lemon zest, lemon juice, cornstarch and granulated sugar.
Add the eggs and whisk till combined.
Transfer the pan to a gas on medium heat and add unsalted butter to the pan.
Keep whisking till butter melts and mixture becomes totally smooth.
After around 10 minutes, the mixture will become really thick and coat the back of your spoon. This happens around 165 F degrees so you can check if you have a kitchen thermometer.
Remove from heat, strain the lemon curd into a bowl and cover with a cling sheet with the sheet touching the surface of the lemon curd so that it doesn't form a film on top.
Refrigerate for few hours.
Fill the cupcakes
Once the cupcakes have baked, take them out of the oven and let them cool down completely.
Once cooled, use a cupcake corer if you have one or like me use a sharp knife to core the cupcake from the center.
Fill the cupcakes with lemon curd. Repeat with all the cupcakes.
Frost with chocolate ganache
Once all the cupcakes are filled, set them aside and make the chocolate ganache.
Add heavy cream in pan on medium heat. Let the cream heat up a bit but do not boil it. Remove it from heat when you see little bubble popping up.
Pour the hot cream over the chopped chocolate. Also add 1/4 teaspoon butter [optional]
Set aside the mixture for 2-3 minutes and then take a spoon and whisk the mixture till chocolate and cream are mixed together completely.
Let the ganache cool down, it will become thick as it cools down.