Preheat oven to 392 F / 200 C, rack in the middle position. Line the 12 piece muffin tin with muffin liners.
In a large mixing bowl of a stand mixer, with flat attachment, mix together the butter and sugar until creamy and smooth, on high speed. It will take about 5 min.
Reduce the speed to medium and add eggs, one by one, and mix them well, until well incorporated. Add also sour cream and vanilla extract and mix until well combined.
In another large bowl, sift in all-purpose flour, baking powder, baking soda, cocoa powder and salt. Whisk with a whisker.
Reduce the speed of the stand mixer to low. And add the dry ingredients into the wet mixture, spoon by spoon. Mix until well incorporate. At the end, the texture will be thick. We will think it with heavy cream. Add 3 tablespoons of heavy cream. Mix until well incorporated.
Add semi-sweet chocolate chips and, with a rubber spatula, incorporate the chocolate chips into the batter. Take the muffin tin with liners and scoop the batter with an ice cream scoop in each muffin liner. There should fit in about 2- 3 scoops, inside one muffin liner.
Bake in preheated oven, middle rack, 395F /200C for 20-25 minutes. Stick a wooden stick at the end of 20 minutes baking to find out if they are ready. If the wooden stick comes out with runny texture, put the muffin tin back to the oven for another 5 min. Once the wooden stick comes out clean, muffins are ready. Take them out of the oven and cool on the wire rack.