Combine the flour, brown sugar, cinnamon, and salt in a small bowl. Add the butter, and mix with a fork or your fingers until the mixture forms crumbs.
Refrigerate while you make the brownies.
To make the brownies:
Preheat oven to 350°F (175°C). Grease an 8-inch square baking pan.
Place the chocolate and butter in a microwave-safe bowl. Heat in the microwave at half power in 30-second increments until the butter is melted and the chocolate melts when stirred. Stir until smooth. Set aside to cool slightly.
Mix the sugar, brown sugar, eggs, and vanilla until well-blended. Add the chocolate-butter mixture, and mix until smooth.
Add the flour and salt, and mix just until combined.
Transfer the batter to the prepared pan and spread evenly.
Sprinkle the crumb topping evenly over the top of the brownie batter.
Bake 35-40 minutes, or until a pick inserted into the center comes out with a few moist crumbs attached. Run a knife along the sides of the pan. Cool in the pan on a wire rack before cutting into bars.
Notes
Keep the crumb topping cold. Refrigerating it while you prepare the brownie batter helps the topping stay crumbly instead of melting into the batter as it bakes.
Don’t over-bake. The brownies are done when a toothpick inserted in the center comes out with a few moist crumbs. They’ll continue to set as they cool.
Cool completely before cutting. This helps the crumb topping firm up and gives you clean slices.