The closest I get is a pencil 1x1, but that's actually drawing a 3x3 cross, with the out areas faded and the center the color you've chosen.

Choosing the pencil with a brush size of 1px actually gives you a 3x3 block
Erasing like it is now is not removing the area instantly, but rather requires many "strokes" before an area is gone. Look at how Photoshop does it instead.

Erasing a pixel actually semi deletes the outer area of a 3x3 block