Preheat the oven to 355° F.
Butter an 8x12-inch baking pan, and line it with parchment paper.
In a heatproof bowl, melt together the butter and the chocolate over a pan of water that’s been brought to a boil and then taken off the heat. Allow the mixture to rest, stirring occasionally as it melts.
2/3 cup unsalted butter cut into small pieces, plus more for the pan, 10 1/2 ounces dark chocolate
In the bowl of a stand mixer, whisk together the sugars, eggs, and vanilla until light and fluffy. Slowly add the melted chocolate, followed by the dry ingredients. Mix just enough to combine.
1/2 cup cocoa powder, 1 1/3 cups whole grain rye flour, 1 cup sugar, 1 cup plus 2 tablespoons light brown sugar, 4 eggs, 1 tablespoon vanilla extract, 1/2 teaspoon baking powder
Pour into the prepared baking pan and smooth the top with a spatula.
Bake for 20-25 minutes, until the brownies are set but with a slight wobble.
Sprinkle with a bit of flaky salt. Leave to cool completely in the pan before slicing into squares. These chocolate rye brownies keep well in an airtight container for a week or so.
1 pinch flaky salt like Maldon, for sprinkling