Preheast oven to 375° F. Spray 4 mini-muffin tins with cooking spray (48 muffins total).
In a bowl, whisk the flour with 1/4 cup cocoa powder. In a separate small bowl, whisk the remaining 1/2 cup cocoa powder with hot water until smooth.
In a large bowl or the bowl of a stand mixer, cream together the sugar, vegetable oil, eggs and yolk, and vanilla on medium speed until combined. Add the cocoa powder/water mixture and beat until just combined. Now alternately add the flour mixture and sour cream in three additions. Scrape down the sides of the bowl and stir in 1 cup of each kinds of chocolate chips, reserving a 1/2 cup of the mini chips to sprinkle on top before baking.
Bake in center of oven about 18-20 minutes or until a toothpick instered in center comes out clean. Cool in pan on wire rack for 5 minutes. Store in an air tight container or freeze in small batches.
This recipe can also be made as one dozen standard sized muffins (30 minute bake time) or 6 jumbo muffins (40 minute bake time).