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Preheat the oven to 300°F. Lightly grease (or line with parchment) two baking sheets.
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Combine the vanilla, baker's ammonia, and salt in a very small bowl. Stir to dissolve the ammonia; the salt won't fully dissolve. Combine the vanilla mixture with the sugar and butter, and beat until smooth.
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Mix in the flour; the mixture will seem quite dry and crumbly at first, but continue mixing until the dough comes together.
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Break off pieces of dough about the size of a shelled chestnut (about 1/2 ounce); a teaspoon cookie scoop works well here. Roll the pieces into balls. Space them on the prepared baking sheets, leaving 1 1/2" between them.
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For somewhat sweeter cookies with the added crunch of sugar topping, roll them in coarse or granulated sugar before placing them on the baking sheets.
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Use the bottom of a glass, dipped in sugar if necessary to prevent sticking, to flatten the balls to about 3/8" thick. If you have it, use the end of the food pusher from a food processor; many of these have a circular, ridged pattern on their end, which imprints the cookies with a nice design.
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Bake the cookies until they're a very light golden brown around the edges, about 25 minutes.
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Remove them from the oven, and transfer them to a rack to cool.
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To dip cookies: Crush the pecans finely, either by putting them in a zip-top plastic bag and pounding, or in a food processor.
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Combine the chocolate chips and vegetable oil in a microwave-safe cup. Microwave till the chocolate softens, then stir till it melts completely.
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Use a pastry brush to pain the bottom of each cookie with chocolate.
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Dip in the crushed nuts.
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Place cookies, chocolate-side up, on a piece of parchment or cookie sheet to set.
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Store shortbread, well wrapped, for up to a week; freeze for longer storage. The flavor improves as the shortbread ages.
