Preheat oven to 375 F. Line an 8 x 8-inch baking pan with parchment paper.
In a medium-sized bowl, whisk sugar, butter, eggs & vanilla.
In a large bowl, whisk flour, cocoa powder, baking powder & sea salt.
Stir the wet ingredients into the dry ingredients & mix until they are just incorporated. Add the rhubarb, chocolate chips & nuts. Mix only until they are combined. The batter will be quite thick. Pour batter into baking pan & spread it out so that it is even.
Bake for 20-25 minutes or just until a toothpick inserted into the middle comes out clean. DO NOT OVERBAKE.
Remove brownies from the oven & allow to cool for at least 10 minutes. Slice into squares.