Preheat oven to 350oF (180oC).
Grease side and bottom of a 7x3 inch. (18x7 cm) round cake pan.Line bottom of pan with parchment paper. In a bowl, add sugar. Then, sift in flour, cocoa powder, baking powder and salt. Mix until combined.
To the same bowl, add water, oil, vanilla extract. Mix to combine. Add red food coloring gel. Mix until no lumps and dry flour no longer visible. Do not over mix. Pour batter into cake pan.
Drop pan on counter to remove any air bubbles.
Immediately, bake in preheated oven at 350oF (180oC) for 35-40 minutes, or until toothpick comes out clean.
Let cake cool down in pan for 10 minutes. Then, transfer to wire rack and let it cool down completely.