Preheat oven to 350 degrees. Lightly oil a 9-inch cake pan, and line bottom of pan with a disk of parchment or wax paper, cut to fit. In a large mixing bowl, whisk eggs with salt, then whisk in sugar and lemon zest. Separately mix dry ingredients (flour, cocoa, baking soda and cinnamon). Sift dry ingredient mixture into egg mixture, gradually folding it in until smooth. Then fold in olive oil a little at a time. Pour batter into pan, and bake for 30 minutes, or until a tester comes out clean. Remove pan from oven and cool on a rack for 10 minutes. Loosen cake from pan with a knife, and invert on rack. Remove paper, invert cake again, and cool to room temperature before serving.