Preheat the oven to 160° C w/ fan.
Line the bottom of the springform w/ baking paper.
Grate orange zest & divide.
Mix flour, baking powder, cocoa & salt.
Mix eggs & sugar w/ an electric handheld mixer until fluffy.
Add the egg-sugar mess to the flour mix & mix.
Add olive oil, milk, orange juice & orange zest - & mix.
Pour the mess into the springform.
Let it bake in the oven for about 40 min.
Make the famous test w/ the wooden pin.
Let it cool down for about 10 min. Remove the springform & transfer to a cooling rack. Invert the cake & peel off the baking paper. Then let it cool down to room temperature.