Set Up Double Boiler: In a small saucepan, add 1 cup of water and place it on high heat.
Melt Chocolate and Butter: Measure out the chocolate and butter into a metal mixing bowl. Place the bowl over the saucepan of water to create a double boiler. Keep the heat on high until the chocolate is fully melted, stirring occasionally.
Mix in Ingredients: Once melted, remove the bowl from the heat and add the sugar, vanilla extract, and eggs. Mix until well incorporated.
Incorporate Sourdough Discard: Add the sourdough discard and mix until fully combined.
Add Dry Ingredients: Sift in the rye flour, cocoa powder, salt, and baking soda. Mix until everything is incorporated, but be careful not to overmix.
Ferment: Cover the mixture and let it ferment for 12+ hours for long fermentation, or proceed to the next step if you're short on time.
Prepare the Baking Dish: Grease and flour a glass 8"x 8" baking dish or line with parchment paper.
Pour Batter: Pour the brownie batter into the prepared baking dish, spreading it evenly.
Bake: Bake at 325°F (163°C) for 30-40 minutes, or until a toothpick inserted into the centre comes out with a few moist crumbs.
Cool and Serve: Sprinkle with Maldon salt and allow the brownies to cool in the baking dish before cutting into squares and serving.