Prep. Preheat oven to 350°F and grease a 12-cup muffin tin with cooking spray.
Melt butter and chocolate. In a double broiler or bane marie, melt butter. Remove from heat and add chopped dark chocolate. Stir together until chocolate is melted. Set aside to cool slightly.
Dry ingredients. In a small bowl whisk together all-purpose flour, cake flour, unsweetened cocoa powder and salt.
Wet ingredients. In the bowl of a stand mixer, beat together sugar and oil. Add eggs and beat until thick and combined, about 3-5 minutes. Add vanilla and beat to combine.
Combine. In 3 additions, add flour mixture alternately with butter mixture beating until just combined. Stir in chocolate chunks.
Add to prepared tin. Add batter in each muffin cup (about 1/3 cup).
Bake. Bake for 18-20 minutes or until tops are puffed slightly and spring back when lightly touched. Cool in pan 10 minutes.