Sift the flour, cocoa, 1 ¼ cups sugar, baking powder, baking soda, and salt into a large bowl; whisk to combine.

Preheat oven to 325°F. In another bowl, whisk egg yolks, oil, water, and vanilla until smooth. Add to dry ingredients and whisk gently until just combined.

In a clean bowl, beat egg whites and cream of tartar on medium speed until foamy. Gradually add the remaining ¼ cup sugar; beat to stiff, glossy peaks.

Fold a third of the whites into the chocolate batter, then fold in the rest until no streaks remain.

Pour into an ungreased 10-inch tube pan, smooth the top, and bake 55-60 minutes until a toothpick has moist crumbs. Invert the pan and cool completely upside down.

Run a knife around the edges, unmold, and slice with a serrated knife.