Prepare a cake pan by greasing it with butter or oil and preheating the oven to 350°F (180°C). In a bowl, combine 1 cup of all-purpose flour, ½ cup unsweetened cocoa powder, baking soda, baking powder, salt, and granulated sugar. Add in one egg, ¼ cup of melted butter or oil, ½ cup buttermilk, warm water (½ cup), and vanilla extract; mix until just combined without overmixing. Pour the batter into the prepared pan and tap lightly before baking for about 30-35 minutes until a toothpick comes out clean. Let cool for fifteen minutes before turning out onto a wire rack to cool completely before slicing.