Preheat the oven to 350ºF (180ºC). Lightly grease two 8-inch (20-cm) round cake tins with olive oil then line them with a circle of parchment paper cut to fit. Set aside.
Place the whole wheat flour, rye flour, cocoa, chopped chocolate, baking powder, baking soda, and salt in a mixing bowl. Whisk to combine. Set aside.
In a second mixing bowl, add the milk, maple syrup, honey, olive oil, eggs, and vanilla. Use a hand or stand mixer to whisk everything together.
Add the flour mixture to the bowl of wet ingredients. Mix everything together on low speed until the flour is just combined (a few streaks of flour is okay).
Divide the chocolate batter evenly between the two prepared cake tins. Place the cakes in the preheated oven and bake for 40 to 45 minutes, or until the sides are starting to pull away and the middle is set.
Remove the cakes from the oven and place the tins on a wire cooling rack to cool for 10 minutes. Turn the cakes out of the tin and onto the wire rack to cool completely before decorating.