In a medium saucepan, melt the butter over medium-low heat.
Add the granulated sugar and stir over low heat to dissolve, about 30 seconds.
Remove from the heat and add the cocoa powder. Whisk or stir to combine.
Add the vanilla extract and 1 egg. Whisk until the egg is completely combined.
Add the second egg and whisk. Add the third egg and whisk until completely incorporated.
Scrape the chocolate mixture into a large bowl.
Add oat flour, ground flax seeds, baking soda, espresso powder, and salt to the large bowl. Use a sturdy spoon to stir the mixture together for 1 minute.
Stir in chocolate chunks. Set batter aside while the oven heats.
Preheat oven to 350F. Spray an 8" x 8" baking pan with cooking spray.
After a 10-minute rest, scrape the batter into the baking pan and bake for 35-42 minutes, or until a toothpick comes out clean.
Let brownies cool for 30 minutes before cutting.
Notes
Use a digital kitchen scale to measure the ingredients.
Use both cocoa and chocolate chips.
Use the right baking pan size to ensure even baking.
Cool the brownies completely before slicing to avoid crumbling.
Doneness CuesThe brownies are ready when:
The edges are firm and slightly puffed.
A toothpick inserted near the center should come out with a few moist crumbs.
The surface loses its wet glisten.
Storage
Room temperature storage: After cooling, keep leftover brownies in an airtight container at room temperature for up to 2 days.
Refrigerating: Wrap the brownies individually in film and refrigerate for up to 5-6 days.
Freezing: Store them in a freezer-safe container or bag for up to 3 months for the best texture.
Thawing: Thaw at room temperature, or microwave frozen ones for 10-15 seconds to bring back that fudgy center.