Melt butter, adding chocolate chips when mostly melted, and melt chocolate and butter together. Stir and allow to cool to room temperature. (I use a large glass measuring cup, and do the butter for about 40 seconds, and then the butter and chocolate together for about 30 more seconds. But your microwave may vary, so proceed with caution.)
Combine flour, sugar, brown sugar, cocoa powder, baking powder, salt, ginger, and pumpkin pie spice in a bowl. Mix well, crushing the brown sugar lumps with a fork until fine.
Add remaining ingredients and mix just until combined. (If you overmix muffins, you get peaky tops and tunnels, instead of a nice, even texture.)
Fold in blueberries, if using.
Bake 20-25 minutes or until a toothpick comes out clean.
Recipe Notes
This batter is quite thick. It's almost fudgy. Don't worry, it's supposed to look like that! Also, your muffin cups will be nearly full. Again, don't worry.