In a small bowl, combine the flour, cocoa, baking powder and salt. Set aside.
In another bowl, mix together the oil, milk, sugars, vanilla, egg and zucchini until well combined. Add in the dry ingredients and mix just until combined. Don't overmix, or your muffins will be tough! Fold in the mini chocolate chips.
For the streusel, combine the flour, sugar, butter, peanut butter and chocolate chips in a small bowl until crumbly.
Spray a jumbo muffin pan with cooking spray. Using an ice cream scoop, put one scoop of batter into each of the cups. Put a tablespoon of streusel, and 8 pieces of chopped Reese's cups over top the batter in each cup. Scoop the rest of the batter evenly over the 6 muffin cups. Batter will be almost to the top of each cup. Sprinkle the remaining streusel evenly over the batter.
Bake at 425 for 5 minutes, then reduce the temperature to 375 and bake for another 20 minutes. Do not open the oven ...just reduce the temperature. A toothpick inserted into the center of a muffin should come out clean. Because of the chocolate in the middle, the toothpick might have a little chocolate on it, but there should not be uncooked batter on it.
Store in an airtight container. These muffins keep for up to 5 days at room temperature.
If you don't have a jumbo muffin tin, you can use a regular muffin tin and get 12 muffins. Reduce the baking time so they do not overbake.