Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners.
In a large mixing bowl, whisk together the all-purpose flour, unsweetened cocoa powder, baking powder, baking soda, and kosher salt until well combined.
In another bowl, combine the hot water and granulated sugar, stirring until the sugar is dissolved. Add in the vegetable oil, eggs, egg yolk, vanilla extract, and sour cream. Whisk until the mixture is smooth and well blended.
Pour the wet mixture into the dry ingredients. Use a spatula to gently fold the ingredients together, being careful not to overmix.
Fold in the semisweet chocolate chips until they are evenly distributed throughout the batter.
Scoop the batter into the prepared muffin tin, filling each cup about 3/4 full.
Place the muffin tin in the preheated oven and bake for 18-22 minutes, or until a toothpick inserted into the center of a muffin comes out clean or with just a few moist crumbs.
Once baked, remove the muffins from the oven and let them cool in the tin for about 5 minutes. Then, transfer them to a cooling rack to cool completely.
