Preheat your oven to 350°F (175°C) and prepare a 9x13-inch baking tin by greasing it with butter or lining it with parchment paper.

In a medium-sized heatproof bowl, melt the unsalted butter and dark chocolate together over a double boiler or microwave until smooth.

Mix in the white granulated sugar and dark brown sugar, then add the eggs and vanilla extract, whisking until thick and glossy.

In another bowl, whisk together the plain flour, Dutch processed cocoa, baking powder, and salt, then sift into the wet mixture.

Pour the brownie batter into the prepared baking tin and smooth the top evenly with a spatula.

Bake for about 30 minutes, checking for doneness with a toothpick.

In a saucepan, combine the white granulated sugar, dark brown sugar, water, and chopped unsalted butter over medium heat. Stir until dissolved, then bring to a boil.

Remove the butterscotch from heat and stir in thickened cream, vanilla extract, and sea salt.

Pour the warm butterscotch sauce over the cooled brownies and let them set at room temperature for at least 30 minutes.

Cut the brownies into squares and sprinkle with sea salt flakes before serving.