Preheat the oven to 350°F and prepare three 4 inch round cake pans. Spray the inside with cooking spray, then cut out pieces of parchment paper so that they fit in the bottom of each pan (do not skip this step!). Set the pans aside.
Add flour, salt, baking powder, and sugar to the bowl of a standing mixer or handheld mixer. Whisk them together until combined.
Add the butter into the bowl of the dry ingredients and beat until the butter is incorporated and the mixture resembles the texture of sand.
Pour in vanilla extract, milk, and oil and mix again. Lastly, add in the egg and mix until just combined (don't over-mix).
Divide the cake batter between the three cake pans and bake for 18-24 minutes, or until a toothpick inserted in the middle of the cakes comes out clean.
Let the cakes cool in the pan for 10-15 minutes, then remove them from the pan (by running a butter knife or offset spatula around the edges and then flipping the cake upside down onto a cooling rack). While the cakes cool, make the buttercream.
Vanilla Buttercream
In the bowl of a mixer, beat the butter (and vegetable shortening if using) together until combined, then add in vanilla extract.
Add in the powdered sugar, ¼ cup at a time until your desired consistency is reached. If the buttercream is too thick, you can add ½ tablespoon of milk in at a time, thinning it out as much as you'd like. If buttercream is too thin, add in more powdered sugar ¼ cup at a time.
Assembling the Cake
Add an even layer of buttercream between each of the completely cooled cake layers.
Frost the outside of the cake, scraping off the excess frosting with a bench scraper (this is called a crumb coat). Then place the cake in the freezer for 20 minutes then add another layer of frosting.
If you'd rather skip the crumb coat, you can simply frost the outside of the cake in one layer then decorate with sprinkles or as desired.
Video
Notes
Please Note: I highly recommend using a kitchen scale to weigh your dry ingredients. Metric measurements are found above under the "Metric" tab.Parchment Lining: I highly recommend lining your cake pans with parchment paper. It's an easy way to avoid the cake breaking when you try to remove it from the pan. To do this, trace then cut a circle in the shape of your cake pan out of the parchment paper and place it inside the pans.Measuring Flour: Either measure your flour using a kitchen scale or use the proper scoop and level method.Dairy Free Butter: If you're using dairy free butter in the buttercream, I recommend using ¼ cup of shortening and ½ cup of butter to help keep the buttercream firmer. Cake Pan Size: This recipe can also be made with two 6 inch round cake pans.Storage and Freezing Instructions:The cake can be stored covered at room temperature for 3-4 days. You can also freeze the cake layers if you want to make this cake ahead of time. Let them cool completely, then tightly wrap each individual cake layer in parchment paper, then aluminium foil. Freeze in ziploc bags for up to 3 months, then let them thaw at room temperature.You can store leftover buttercream in an airtight container in the freezer for up to 2 months. Thaw it in the fridge then re-whip it using a handheld or standing mixer before frosting the cakes.