Preheat the oven to 350°F. The oven should be fully hot before baking.
In a bowl, mix the softened butter and sugar until smooth and pale. It should look creamy, not grainy.
Add the egg and mix until fully blended. The mixture should look glossy and even.
Add the flour, baking powder, and salt. Mix just until a soft dough forms. It should hold together and not feel sticky.
Scoop the dough into balls and place them on the baking sheet. Leave space so they can spread slightly.
Bake for 10–12 minutes. The cookies should look set with a very light color on the edges.
Let the cookies rest on the pan for a few minutes, then move them to a rack. They will firm up on the outside as they cool.