I always start by turning my oven to 350°F so it's fully heated by the time my batter is ready. I grease my 8x8 pan with butter or a light coating of cooking spray, then dust it lightly with flour so the cake releases cleanly once it's baked.
In my mixing bowl, I beat the softened butter and sugar together until the mixture turns pale and a little fluffy. This step takes about two minutes and it's what gives the finished cake its tender, light texture instead of a dense one.
Next, I crack in my egg and pour in the vanilla extract, beating just until it's fully combined. I like to stop here and scrape down the sides of the bowl so nothing gets left out of the mix.
I whisk my flour, baking powder, and salt together in a separate small bowl first. Then I add it to the butter mixture in two batches, alternating with the milk, stirring gently so I don't overwork the batter.
If I'm using chocolate chips or nuts, this is where I fold them in with a spatula using slow, gentle strokes. I pour the finished batter into my prepared pan and smooth the top so it bakes evenly.
I slide the pan into the oven and bake for 28 to 32 minutes, checking at the 25-minute mark. The cake is ready when a toothpick inserted in the center comes out clean or with just a few moist crumbs.
Once it's out of the oven, I let the cake cool in the pan on a wire rack for at least 15 minutes. Cutting into it too early makes the squares crumble, so I always make myself wait it out.