Preheat the oven to 180 C, 160 Fan, Gas 4. Butter a 20 cm by 30 cm baking tin and line with parchment paper.
In a heatproof bowl melt together the 150g butter and the chocolate over a pan of water that has been brought to the boil and then taken off the heat. Allow the mixture to rest, stirring occasionally, as it melts.
In another bowl whisk together the cocoa, rye flour, baking powder and salt.
In the bowl of an electric mixer mix 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, then pour into the prepared baking tin. Smooth the top with a palette knife then sprinkle with a tsp or so of big flaky sea salt flakes.
Bake for 20-25 minutes until the brownies are set but with a slight wobble. Leave to cool completely in the tin before cutting into 12 thick but smallish squares.