Preheat the oven to 350F.

Butter and line a 9-by-13-inch metal baking dish with parchment paper

In a small saucepan, melt the butter over low heat.

Remove from the heat and add the chocolate. If the heat from the butter does not fully melt the chocolate, put the pan back over the heat for 10 seconds and stir until melted.(note: if you are using a chocolate bar, you will need to melt the chocolate longer on the stove)

Set chocolate mixture aside to cool.

Sift the flour and cocoa powder into a small mixing bowl. Set aside.

In a medium mixing bowl, combine the eggs, sugars, salt, and vanilla. Using a mixer fitted with the whisk attachment (or hand mixer), beat on high speed until the mixture thickens and becomes pale in color and falls from the beater in a wide ribbon that folds back on itself and slowly dissolves on the surface, 4 to 5 minutes.

Using a rubber spatula, fold the cooled chocolate into the egg mixture.

Add the flour and cocoa powder and fold it in quickly but gently with the rubber spatula so that you don’t deflate the air that’s been incorporated into the eggs.

Pour the batter into the prepared dish and smooth the top with a spatula.

Bake until the top looks slightly cracked and feels soft to the touch, about 25 minutes.

Set aside to cool.

In a medium pot, combine the sugar, water, salt, and corn syrup (use a pan larger than the contents as it will bubble to ~4x the contents) Bring to a boil over medium heat, stirring to dissolve the sugar. Then cook, without stirring, until the mixture is amber colored, 5 to 8 minutes. Remove from the heat. (Complete step 2 while the mixture is becoming an amber color)

While the sugar is heating, pour the heavy cream into a small pot. Add the vanilla bean (split and seeded with the tip of a sharp knife) to the cream. Place the pan over medium-high heat until it is just under a boil. Reduce the heat to low to keep the milk warm.

Slowly pour the warmed cream into the amber sugar syrup. At first, the mixture will boil furiously but then will simmer down. Whisk until smooth. Add the lemon juice and cool for 10 minutes.

One tablespoon at a time, add the butter to the caramel. Whisk constantly after each addition. Cool for about 10 minutes before adding in the peanuts.

Add the peanuts to the caramel and stir to evenly combine before pouring over top of the cooled brownies. Sprinkle with flaky sea salt and let them set up for about 30 minutes before slicing.