Preheat the oven to 325 F (160 C). Butter a loaf pan and line with parchment paper. We like to lay a piece of parchment paper across the pan for easy removal after baking!
Add softened butter and sugar to the bowl of a stand mixer.
Cream together the butter and sugar until very light and fluffy, using the paddle attachment. Make sure to scrape down the sides a few times.
Make sure the eggs are room temperature, so that your batter doesn't split! If your room is quite cold, or you forgot to put the eggs out earlier, place the eggs in a bowl of warm water for around ten minutes and they will warm up. Add one egg and run the mixer until fully incorporated, then scrape down the sides of the bowl.
Repeat with the remaining two eggs, adding them one at a time and mixing well. If you find that your batter is looking split/curdled, try to keep mixing on high speed after scraping down the sides. If it's still split, try adding a few tablespoons of flour to get it to hold together.
Add the vanilla extract to the eggs, butter, and sugar mixture. Mix together.
Measure out the milk in a bowl/cup and mix the flour, baking powder, and salt together in a separate bowl.
Sift in half of the dry ingredients and mix together until just incorporated (probably around 10-15 seconds). In place of sifting, you can whisk your dry ingredients together very well before adding to get rid of any lumps.
Then add half of your room temperature milk and mix until incorporated.
Sift in the remaining dry ingredients, mix, then add the remaining milk and mix again.
Once incorporated, give the mixture a few folds with a spatula to make sure everything from the bottom has been mixed.
Now, split the batter evenly into two bowls.
Add cocoa powder into one of the bowls and mix together with a spoon until fully combined. Set aside.
With the other half of the batter, add in orange zest from one orange or about 1/2 tsp of orange extract (the extract goes a long way, so be careful of adding too much!).
Then add in orange food coloring (this will be so dependent on the type of food coloring you use, but we added together yellow and red natural food coloring until it became a light orange color! Feel free to use whatever you like, but it may need way less or a little more coloring depending on the type you use!).
Mix the batter together until you get to the orange color you like.
Now we're going to mix the batter together in the loaf tin to create a marbling effect. On the bottom layer, add one third of the orange batter, but in three offset dollops (top left corner, middle right section, and bottom left corner).
Then add the chocolate batter in the spaces where you didn't add the orange batter - top right corner, middle left, and bottom right corner. You should have an alternating pattern of batter on this bottom layer.
Then add the next third of the orange batter on top of the chocolate batter that you just added.
Add the next third of the chocolate batter on top of the orange batter (the layer on the bottom, not the layer you just added).
Repeat one last time with the remaining third of the two batters, adding the chocolate on top of the orange and the orange on top of the chocolate.
Tap the tin on the counter a few times to distribute the batter evenly.
Using a skewer or a knife, make a zigzag pattern back and forth horizontally all the way down the cake.
Remove the knife, wipe it off, and make a few vertical zigzag lines as well.
Tap the pan against the counter a few times again.
Bake for 55 minutes (could take up to an hour to an hour and ten minutes if not using convection settings), or until a cake tester/skewer comes out clean. Remove the cake from the oven, let sit for about 5 minutes, then remove from the pan and place on a cooling rack.
Once cool, slice and enjoy!