Preheat the oven to 350°F (177°C). Place paper cupcake liners in all 12 wells of a standard muffin pan to prevent sticking and allow the baked cupcakes to release easily.

Place the softened unsalted butter and granulated sugar in a large mixing bowl. Using a handheld mixer or a stand mixer fitted with the paddle attachment, beat the ingredients at medium-high speed for approximately 2 minutes, or until the mixture becomes pale, airy, and fluffy.

Pause the mixer and scrape the sides and base of the bowl to ensure the ingredients are evenly blended.

Add the room-temperature eggs and vanilla extract to the creamed butter mixture. Beat at medium-high speed for about 1 minute, continuing until the eggs are fully incorporated.

Scrape the bowl once more so that no unmixed portions remain around the edges or bottom.

In a separate medium-sized bowl, thoroughly whisk together the all-purpose flour, baking powder, and salt.

Reduce the mixer to low speed. Gradually add the dry mixture to the butter-and-egg mixture.

Slowly pour in the whole milk, fresh lemon juice, and finely grated lemon zest. Continue mixing only until the batter is uniform and no visible streaks of flour remain. Avoid excessive mixing, as this may make the cupcakes dense rather than light and tender.

Divide the prepared batter evenly among the lined muffin wells, filling each liner approximately two-thirds full.

Bake standard-sized cupcakes for 18–22 minutes. The cupcakes are ready when a toothpick inserted into the center comes out clean.

For mini cupcakes, reduce the baking time to 11–13 minutes.

Remove the pan from the oven and allow the cupcakes to cool completely before applying the frosting.

Once the cupcakes have cooled fully, cover each one generously with vanilla buttercream frosting.

Garnish with fresh lemon slices, if using. A light scattering of additional lemon zest may also be added immediately before serving.