Whisk brown sugar and oil in a large bowl until combined. Add eggs and vanilla and whisk until combined. Combine rye flour, all-purpose flour, baking powder and salt in a medium bowl. Add the dry ingredients to the wet ingredients, stirring until just combined. Fold in 1/2 cup chocolate. Scrape the mixture into the prepared pan and sprinkle with the remaining 1/4 cup chocolate and flaky salt.