Preheat oven to 350 F (176 C).
Line an 8x8 inch square baking pan with parchment paper, let paper hang over sides. If you choose to use a 9x9 inch baking pan, the brownies will cook in shorter time, so check them after 30 minutes of baking.
In a medium bowl, hand whisk together flour, cocoa powder, cinnamon, espresso powder and sea salt. Set aside.
Using a separate medium bowl combine dark chocolate chunks with the hot melted butter and stir with a fork until all of the chocolate is melted and you have a shiny buttery liquid. Then, stir in the vanilla extract. Let cool slightly to room temperature.
Place granulated sugar and dark brown sugar in a large bowl. Add the eggs and egg yolk, then whisk together with an electric hand mixer on medium high until foamy and thick, around 2-3 minutes.
Pour the chocolate mixture into the sugar and eggs. Fold together using a rubber spatula in soft figure eights until just combined.
Spoon in a third of the flour mixture into the chocolate mixture, folding with spatula until in a figure eight pattern, and around the sides of the bowl until just mixed. Repeat with the remaining flour mixture. The batter will be thick and shiny.
Scoop batter into the prepared brownie pan and level with an offset spatula.
Bake on the middle rack for 45-48 minutes. Check the brownies after 35 minutes of baking. Brownies will be set on the edges and look soft in the middle, but not uncooked. A toothpick inserted into the center should come out with just a few wet crumbs. The residual heat of the pan will continue to cook the center of the brownies, which will firm as they cool.
Let brownies completely cool before lifting from pan, slicing and serving. Sprinkle with flaky sea salt!