Preheat the oven to 350°F / 175°C. Generously grease as many 8-inch / 20cm cake pans as you have, then line them with greased parchment paper. (To bake the nine layers, you will have to bake multiple batches. The more pans you have, the faster this will go).
In a medium-sized pot heat the honey over medium heat until just simmering around the edges. Turn off the heat, add the baking soda, stirring constantly until the honey foams, thickens and turns from light color to a caramel color, about 2 minutes. Transfer into a bowl to cool.
In a stand mixer fitted with the paddle attachment, cream the butter on medium-high speed until creamy and smooth, about 1 minute.
Turn the mixer speed to medium; add the oil, sugar, and vanilla to the butter; and beat for about 5 minutes, until the mixture is light in color and about double in volume.
Turn the speed to medium-low and add the eggs and egg yolks, one at a time, mixing each until incorporated before adding the next. Scrape the bowl after each addition.
In a medium bowl, whisk together the flour, baking powder, and salt until well combined. In a liquid measuring cup, whisk together the milk and sour cream.
Add one-third of the flour mixture to the butter mixture and mix until combined. Scrape the bowl and add half of the milk mixture, mixing until combined. Repeat with another one-third flour, the remaining milk mixture, and then finish with the final one-third flour, scraping the bowl and paddle after each addition.
Fold in the cooled honey mixture.
Pour 150g (3/4 cup) of the batter into the prepared pans and spread evenly using an offset spatula. Gently tap the pans on the counter to release excess air bubbles.
Bake until golden and set, about 12-14 minutes. Remove the cakes from the pans, cool completely on a wire rack. Line the pans again with greased parchment and bake the next batch of cakes. Repeat until you have baked all of the batter. If you are not weighing the batter, you may get more or less layers.