Take an 8x8-inch (20cm square) baking pan and line it with baking parchment paper.
In a large mixing bowl, sift together the flour, sugar, cocoa powder, baking soda, and salt.
Create three wells in the flour mixture. Make one large well and two smaller ones.
Pour the oil into the large well, vinegar into one of the smaller wells, and vanilla into the third well. Note: It's important to keep the wet ingredients separate until they are stirred into the batter simultaneously to ensure the cake turns out properly.
Add the cold coffee and water into the mixing bowl, and mix everything until well-blended without any lumps.
Pour the batter into the prepared pan.
Place the pan in the preheated oven and bake for approximately 25 minutes, or until a toothpick inserted into the center of the cake comes out clean.
Allow the cake to cool in the pan.
Once the cake has cooled, coat it with frosting of your choice.
Cut the cake into slices and serve.
Notes
Sifting the flour, cocoa powder, and baking soda helps to remove any lumps and ensures an even distribution of ingredients. It leads to a smoother batter and a lighter texture in the final cake.
Keeping the wet ingredients separate until they are added simultaneously into the batter is crucial. This method allows the vinegar and baking soda to react at the right time, resulting in a well-risen and tender cake.
Once the cake batter is mixed, don't leave it long before baking. Ensure the oven is preheated before making the batter.
Allow the cake to cool completely in the pan before frosting or cutting it