Step 1: Preheat your oven to 175°C (350°F). Get three 8-inch round cake pans ready. Grease them well, then line the bottoms with parchment paper. Grease the parchment too.
Step 2: In a big mixing bowl (or the bowl of your stand mixer), beat together the softened butter and granulated sugar until the mixture is light and fluffy (3-5 minutes). Scrape down the sides as needed.
Step 3: Add your eggs one at a time, beating well after each is fully incorporated.
Step 4: Stir in the vanilla extract.
Step 5: In a separate medium bowl, whisk together the flour, baking powder, salt, and orange zest. Set aside.
Step 6: In a small bowl or jug, whisk together the fresh orange juice and whole milk.
Step 7: Add the dry ingredients and the milk/juice mixture to the butter mixture, alternating between them. Start with about a third of the dry ingredients, mix on low until mostly incorporated. Add half of the milk/juice, mix until combined. Repeat with the second third of dry ingredients, then the rest of the milk/juice, and finally the last third of dry ingredients. Mix only until no streaks of flour remain. Do not overmix.
Step 8: Divide the batter evenly among your three prepared cake pans.
Step 9: Bake for 25–30 minutes, or until the edges pull away and a toothpick inserted into the center comes out clean.
Step 10: Let the cakes cool in their pans on a wire rack for about 10-15 minutes.
Step 11: Carefully flip the cakes out onto the wire rack to cool completely. Remove the parchment paper. Ensure they are absolutely cold before frosting.
Step 12: While the cakes cool, make the frosting. In a large mixing bowl, beat softened butter until smooth and creamy.
Step 13: Gradually add powdered sugar, beating on low speed about a cup at a time until incorporated.
Step 14: Once all the powdered sugar is in, add the vanilla extract, 1 tablespoon of fresh orange juice, and the orange zest.
Step 15: Beat the frosting on medium-high speed for 3–4 minutes until light, fluffy, and smooth. If too thick, add the second tablespoon of orange juice, a teaspoon at a time, beating until desired consistency.
Step 16: Once cakes are completely cool, place one layer on your cake stand. Spread a generous layer of frosting evenly over the top.
Step 17: Carefully place the second cake layer on top and repeat with another layer of frosting.
Step 18: Add the third cake layer on top.
Step 19: Apply a thin crumb coat layer of frosting all over the top and sides of the cake using an offset spatula.
Step 20: Chill the cake in the refrigerator for 15–20 minutes to set the crumb coat.
Step 21: Apply a final, thicker, smooth layer of frosting over the sides and top of the cake.
Step 22: Optionally, pipe decorative dollops around the top edge of the cake with additional frosting or whipped cream.
Step 23: Arrange fresh orange slices around the base and/or on top for decoration. Slice and serve immediately.