Preheat oven to 350 degrees. Lightly spray 4 - 12 oz ceramic mugs.
Place ¼ cup (half a stick) of butter and the chocolate chips into a small glass bowl. Heat in the microwave approx. 30–45 seconds until the butter has melted. Stir to melt the chocolate chips. If the chocolate doesn’t completely melt, return to the microwave and heat in 15-second intervals until completely melted. Set aside to cool just a bit.
Sift cocoa, flour, baking soda and baking powder together in a small bowl and set aside.
Cream together remaining ¼ cup of butter, sugar, vanilla and salt in a large mixing bowl, or the bowl of a stand mixer fitted with the paddle attachment. Add the eggs one at a time, blending the first one completely before adding the next. Add the melted, cooled chocolate then stir to combine.
Add ⅓ of the dry ingredients then stir until completely incorporated. Add sour cream and stir to combine. Repeat with another ⅓ of the dry ingredients. Finish by adding the milk, then the remaining dry ingredients, stirring each addition just until combined.
Divide batter evenly among the mugs. Place mugs on a rimmed baking sheet and bake 12-15 minutes, or until the edges look done and are firm to the touch and the middle is soft and moist. (You want these to be slightly underdone.) Transfer cakes to a wire rack to cool slightly. When the centers of the cakes have collapsed (about 5 minutes later), coat the indentation with a sprinkling of powdered sugar and a few berries. Dig in!
One Last Thing
If you're feeling extra decadent, serve these lava cakes with a heaping scoop of vanilla ice cream!