In a large shallow saucepan, bring water to a gentle simmer. Add sugar and dissolve completely, then add sliced oranges. Simmer for 15 minutes, flip the slices, and simmer for another 15 minutes until tender. Remove oranges and cool on a drying rack.

Preheat your oven to 350°F (177°C) for convection (or 325°F/162°C for conventional). Grease four 6-inch cake pans (or two 8-inch pans) to ensure easy release. Grate the zest from 5 blood oranges, reserving 3 for the buttercream.

Juice the 5 blood oranges using a citrus juicer. Reserve 1/4 cup of juice for the buttercream, and mix 3/4 cup with the olive oil and zest for your batter.

In a large bowl, whisk together flour, sugar, cocoa powder, salt, and baking soda until well combined. Fold in the olive oil mixture and water until smooth.

Divide the batter evenly among the prepared pans and bake for 20-25 minutes, or until a toothpick inserted in the center comes out clean.

In a stand mixer, beat the butter with the reserved blood orange zest until pale and fluffy. Gradually add half of the powdered sugar, mixing until incorporated, then add the remaining sugar and beat for 5-10 minutes until smooth.

With the mixer on low-medium speed, slowly pour in the vanilla and blood orange juice, mixing until fully combined and creamy.

Place dark chocolate in a medium bowl. In a saucepan or microwave, heat the half and half until bubbling, then pour it over the chocolate. Let sit for 1 minute, then stir until smooth.

Allow the ganache to cool for 5-10 minutes before pouring it over the cake for delicious drips. If it cools too much, microwave it for 10 seconds and stir to reconstitute.

Begin with the first cake layer on a flat surface, evenly spreading about 1/4 of the buttercream on top. Repeat with the remaining layers, using leftover buttercream to frost the outside. If the cake feels unsteady, refrigerate for 15-30 minutes to set.

Finally, refrigerate for an additional 30 minutes before pouring the chocolate ganache on top, letting it set for another 15 minutes. Garnish with candied orange slices if desired.