Preheat oven to 350°F (175°C). Grease and flour the 6-inch cake pan, then line the bottom with parchment paper. This triple preparation ensures your cake will release cleanly after baking.
In a medium bowl, whisk together the sifted cake flour, baking powder, baking soda, and salt. Set aside. This pre-mixing helps distribute the leavening agents evenly throughout the cake batter.
150 g Cake flour, 1.5 tsp Baking powder, ¼ tsp Baking soda, ¼ tsp Salt
In a large bowl using the electric mixer, beat the softened butter on medium speed until creamy (about 1 minute). Add the granulated sugar and beat on medium-high speed until light and fluffy (about 3 minutes), scraping down the sides of the bowl as needed. The mixture should look pale yellow and have increased in volume – this step incorporates air that helps the cake rise.
113 g Unsalted butter, 150 g Granulated sugar
Beat in the room temperature egg until completely combined. Then, beat in the vanilla extract. The mixture should be smooth and well-incorporated after this step.
1 large Egg, 2 tsp Pure vanilla extract
With the mixer on low speed, add about one-third of the dry ingredients, followed by about half of the milk. Mix until just combined after each addition. Repeat, adding another third of the dry ingredients, the remaining milk (and oil if using), and finally the last third of the dry ingredients. Crucial: Mix only until the last streaks of flour disappear in the cake batter. Do not overmix, as this will develop gluten and make your 6 inch cake tough rather than tender.
150 g Cake flour, 1.5 tsp Baking powder, ¼ tsp Baking soda, ¼ tsp Salt, 120 ml Whole milk, 1 tbsp Vegetable oil
Pour the cake batter evenly into the prepared 6-inch cake pan. Smooth the top with the rubber spatula to ensure even baking and a level cake layer.
Bake in the preheated oven for 30-35 minutes. Your cake is done when a wooden skewer or toothpick inserted into the center comes out clean, the top springs back lightly when touched, and the edges are pulling away slightly from the pan. For absolute certainty, the internal temperature should reach approximately 210°F (99°C).
Let the cake cool in the pan on a wire rack for 10-15 minutes. Then, carefully invert the cake onto the wire rack, remove the pan and parchment paper, and let it cool completely before frosting. Complete cooling is essential as frosting a warm cake will cause the frosting to melt and slide off.
For the vanilla buttercream (if using): Beat butter until creamy, then gradually add powdered sugar, milk, vanilla, and salt. Beat until light and fluffy. This makes enough vanilla buttercream for a simple coating and some basic piping for your 6-inch cake.
113 g Unsalted butter, 240 g Powdered sugar, 1-2 tbsp Milk or heavy cream, 1 tsp Vanilla extract, 1 pinch Salt