Preheat your oven to 180°C (350°F). Grease your cake tin and line it with parchment paper or butter.
Peel and finely grate the beetroots, using either a grater or food processor. You should have about 1 to 1.5 cups of grated beetroot.
In a medium bowl, sift together the flour, cocoa powder, baking powder, salt, and cinnamon powder (if using). Set aside.
In a large bowl, whisk together the sugar, eggs, vegetable oil (or melted butter), vanilla extract, and milk. Add the grated beetroot and stir to combine.
Gradually fold the dry ingredients into the wet mixture, using a spatula or electric mixer, until the batter is smooth and well mixed.
Stir in the melted dark chocolate and lemon juice (or vinegar) until fully incorporated.
Pour the batter into the prepared cake tin and bake for 35-40 minutes, or until a skewer inserted into the center comes out clean. Let the cake cool completely in the tin before removing it.