Preheat oven to 180°C (350°F) and grease a cake tin.
Peel beetroot and puree until smooth using a food processor or blender.
Melt dark chocolate using double boiler or microwave, then mix with beetroot puree.
In a large bowl, beat eggs, caster sugar, vegetable oil, and vanilla extract until smooth.
In a separate bowl, combine flour, cocoa powder, baking soda, and salt.
Gently fold dry ingredients into wet ingredients until just combined.
Fold in the beetroot-chocolate mixture until evenly distributed.
Pour batter into prepared cake tin.
Bake for 60 minutes or until a skewer inserted in center comes out clean.
Cool completely before frosting or serving.