1. Whisk the flour, salt and baking powder together.
2. Working with a stand mixer, preferably fitted with a paddle attachment, or with a hand mixer in a large bowl, beat the butter at medium speed for a minute or so, until smooth. Beat in the sugar and continue to beat for about 2 minutes, until the mixture is light and pale. Add the egg and yolk and beat for another minute or two; beat in the vanilla. Reduce the mixer speed to low and steadily add the flour mixture, mixing only until it has been incorporated — because this dough is best when worked least, you might want to stop the mixer before all the flour is thoroughly blended into the dough and finish the job with a rubber spatula. When mixed, the dough will be soft, creamy and malleable.
3. Turn the dough out onto a counter and divide it in half. If you want to make roll-out cookies, shape each half into a disk and wrap in plastic. If you want to make slice-and-bake cookies, shape each half into a chubby sausage (the diameter is up to you — I usually like cookies that are about 2 inches in diameter) and wrap in plastic. Whether you're going to roll or slice the dough, it must be chilled for at least 2 hours. (Well wrapped, the dough can be refrigerated for up to 3 days or frozen for up to 2 months.)
4. Center a rack in the oven and preheat the oven to 350 degrees F. Line two baking sheets with parchment or silicone mats.
5. If you are making roll-out cookies, working with one packet of dough at a time, roll out the dough between sheets of plastic wrap or wax paper to a thickness of 1/4 inch, lifting the plastic or paper and turning the dough over often so that it rolls evenly. Lift off the top sheet of plastic or paper and cut out the cookies — I like a 2-inch round cookie cutter for these. Pull away the excess dough, saving the scraps for rerolling, and carefully lift the rounds onto the baking sheets with a spatula, leaving about 1 1/2 inches between the cookies. (This is a soft dough and you might have trouble peeling away the excess or lifting the cutouts; if so, cover the dough, chill it for about 15 minutes and try again.) After you've rolled and cut the second packet of dough, you can form the scraps into a disk, then chill, roll, cut and bake.
6. If you are making slice-and-bake cookies, use a sharp thin knife to slice the dough into 1/4-inch-thick rounds, and place the rounds on the baking sheets, leaving about 1 1/2 inches of space between the cookies.
7. Bake the cookies one sheet at a time for 9 to 11 minutes, rotating the sheet at the midpoint. The cookies should feel firm, but they should not color much, if at all. Remove the pan from the oven and dust the cookies with sugar or cinnamon sugar, if you'd like. Let them rest for 1 minute before carefully lifting them onto a rack to cool to room temperature.
8. Repeat with the remaining dough, cooling the baking sheets between batches.
Storing: The cookies will keep at room temperature in a tin for up to 1 week. Wrapped well, they can be frozen for up to 2 months.
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