1. Preheat the oven to 160°C.
2. For the cake, spray two 9-inch cake pans with cooking spray and sprinkle cocoa on the inside of each pan, tapping evenly to coat.
3. Combine the beans, vanilla extract, salt, stevia and 6 of the eggs in a blender. Blend on high until the beans are completely liquefied. In a separate bowl, whisk together the cocoa powder, baking powder and baking soda.
4. Beat the butter with the honey in a stand mixer on high speed until light and fluffy. Add the remaining 4 eggs one at a time, beating for a minute between each addition. Pour the bean batter into the egg mixture and mix until smooth. Stir in the cocoa powder mixture and beat the batter for one minute. Pour the batter into the prepared cake pans and smooth the tops.
5. Bake the cakes until a toothpick inserted into the center of each comes out clean and the cake springs back to the touch, 40 to 45 minutes. Let the cakes cool in the pans for 10 minutes, then flip them out onto a cooling rack to cool to room temperature.
6. For the frosting, beat the butter in a stand mixer on medium speed until light and fluffy, a few minutes. Reduce the speed to the lowest setting and add 3 cups of the confectioners' sugar and beat until incorporated. Increase the speed to medium and add 3 tablespoons of the milk, the vanilla extract and salt and beat for 3 minutes.
7. Mix in the cocoa powder and blend. If the frosting is a little thin, add the remaining sugar in batches. If it is too thick, add more milk a tablespoon at a time.
8. Frost the cakes.