Preheat the oven to 400°F. Grease a 9" square baking pan and then line it with parchment paper.
Using a double boiler, melt the butter and baking chocolate together, mixing every so often. Once it's fully melted and combined, remove it from heat and set it aside to cool to room temperature.Note: if you don't have a double boiler, you can make an impromptu version by placing a heat proof bowl over a pot of boiling water. Just make sure the water doesn't touch the bowl. Using an electric mixer, whisk together the 2/3 cup granulated sugar, 2/3 cup light brown sugar, and 3 full eggs, along with an extra yolk (reserve the white for the blue cheese mixture) on medium speed. You'll know it's done when it's light and creamy.
Add the chocolate sauce to the sugar and eggs and stir to combine.
Gently mix in the cocoa powder, flour, and salt. Stop mixing once combined.
Pour the brownie batter into your greased and lined pan.
Now add the blue cheese, 1/4 cup granulated sugar, and 1 egg white to the bowl of a food processor and blitz until smooth and combined.
Spoon dollops of blue cheese mixture onto the top of the brownie batter. Use a toothpick to create nice little swirls. Keep doing this until you're satisfied with the swirly pattern on top! The more blue cheese mixture you use, the stronger that taste will be, so use your judgment here.
Bake for 23-25 minutes, or until the middle is a little bit wobbly. They might seem underbaked at first, but this is actually ideal if you're after a perfectly fudgy texture. They'll also continue to firm up as they sit.
If you want to serve these while they're warm, do it. They might be a little hard to handle, but they'll be absolutely delicious. If you prefer a slightly firmer brownie, wait until they reach room temperature before cutting into bars.