Preheat oven to 350° F. Use non-stick cooking spray to generously grease a bundt pan or round tube pan.
Melt the butter and allow to cool.
In a small bowl, mix together 1 Tablespoon of the total amount of sugar with the cream of tartar. Set aside.
Whisk together the flour, cocoa powder, sugar (less 1 Tablespoon), baking powder, and sea salt.
With the mixer on medium speed, whisk the egg whites until foamy. Once it is fully foamy, add the sugar/cream of tartar mixture, slowly, as the whites continue to whisk. Continue whisking until stiff peaks form. (You can increase the mixer to high towards the end.)
In a large mixing bowl, mix the oil and the vanilla into the cooled brown butter. Fold approximately ¼ of the egg whites into the butter mixture. Once combined, add the rest of the stiff egg whites on top, then pour the dry ingredients on top of the stiff egg whites. Gently fold all the ingredients together just until fully combined.
Bake for 20-30 minutes. A toothpick inserted in the middle should come out dry, and the edges should just start pulling away from the sides of the pan. Cool in pan for 10 minutes, then invert onto a wire rack to cool completely.