Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking pan.

In a large mixing bowl, cream together the unsalted butter and granulated sugar until light and fluffy.

Add the eggs one at a time, beating well after each addition.

Stir in the vanilla extract.

In a separate bowl, sift together the all-purpose flour, baking powder, and salt.

Gradually add the dry ingredients to the butter-sugar-egg mixture in three parts, alternating with the whole milk. Stir just until incorporated.

Ensure the batter is smooth and uniform.

Pour the batter into the prepared baking pan and spread it evenly.

Bake for 35-40 minutes, or until a toothpick inserted into the center comes out clean.

Allow to cool in the pan for about 10 minutes before transferring to a wire rack.

Cut into squares or slices and serve.