Preheat and prepare: Preheat oven to 325°F. Generously butter a 9×9-inch baking dish. Set aside. Melt the butter and let it sit to cool.
Whip the eggs and sugar: In a stand mixer fitted with the paddle attachment, beat the eggs and sugar on medium-high speed for 5–10 minutes, until the mixture becomes pale, thick, and ribbon-like. This step builds the structure of the dessert and helps it set properly during baking.
Combine the dry ingredients: In a separate bowl, sift together the cocoa powder, flour, and salt to ensure even distribution and prevent clumping.
Add the flavor and color: Reduce the mixer speed to low. Add the buttermilk, vanilla extract, vinegar, and red food coloring. Mix just until evenly incorporated and the batter is uniformly red.
Incorporate the dry ingredients: Add the sifted dry ingredients to the mixer and mix just until combined. Avoid overmixing.
Add the butter: Slowly stream in the melted butter while mixing on low speed until fully incorporated. The batter will be loose and glossy.
Prepare the water bath: Pour the batter into the prepared 9×9 dish. Place the baking dish inside a larger roasting pan. Carefully pour hot water into the outer pan until it reaches halfway up the sides of the baking dish.
Bake: Bake for 55–65 minutes. Begin checking at 55 minutes. The edges should be set, and the center should jiggle like thick pudding. A toothpick inserted into the center should come out with moist crumbs, not liquid batter.
Cool and serve: Remove the dish from the water bath and allow it to cool for 10–15 minutes. Serve warm, scooped into bowls, and top with ice cream, berries, powdered sugar, or cream cheese glaze.