These delicate French-style butter cookies melt in your mouth--a surprise, given oat's reputation for sturdiness. For more about the ingredients, see "Power Flours," below.
Put almonds, almond extract, and about 2 tbsp. rice flour in a food processor and whirl until almonds are very finely ground. Add remaining rice flour, the oat flour, granulated sugar, salt, and baking soda. Pulse to blend thoroughly. Add cream cheese and butter, cutting both into chunks as you add them. Pulse just until mixture forms a smooth dough. "If you work it too long, the cookies will be too crumbly." Scrape bowl and blend in any stray flour at the bottom with your fingers.
Turn dough out onto a sheet of waxed or parchment paper and form it into a rough log about 10 in. long. Lay a long side of the waxed paper over dough log and set a ruler lengthwise against the log; then press the ruler against the log, squeezing it until it's smooth (it will get longer). Twist ends to seal. Chill until firm enough to handle, 30 minutes, and roll into a rounder shape. Chill at least 3 1/2 hours more (preferably overnight).
Position racks in upper and lower thirds of oven and preheat to 325°. Line 2 large baking sheets with parchment, or butter them.
Unwrap dough and roll log in coarse sugar to coat it completely. Use a thin, sharp knife to cut 1/4-in.-thick slices. "It's good to use a ruler, because thinness makes a difference in baking time and texture. Also, if the dough is cracking, don't panic. Just let it warm up at room temperature for a few minutes before slicing." Place cookies about 1 in. apart on baking sheets.
Bake cookies, rotating pans from top to bottom and from front to back halfway through. "This is so important because the heat in ovens isn't even." Bake until golden brown at the edges and well browned on the bottom, 20 to 25 minutes. "Most people underbake their cookies. You want them to get a toasty flavor."
Set pans on cooling racks and let cool completely.
Make ahead: At least 2 weeks at room temperature in an airtight container.
Each flour has its own character, so it doesn't always work to substitute. Also, it's key to weigh the flours--they can fluff up or settle, which throws off a volume measure. (For more, see sunset.com/weighing.) Find at well-stocked grocery stores, bobsredmill.com, or authenticfoods.com.
White Rice Flour: Milled from rice that's had the bran removed. Produces fine-crumbed cakes and silky puddings; amplifies flavors.
Brown Rice Flour: Has the structure-building properties of white rice flour, plus a slight graininess and toasty caramel flavor.
Oat Flour: Powdered oatmeal, with a sweet, butterscotchy flavor. Makes tender cakes and delicate cookies.
Buckwheat Flour: From roasted whole buckwheat seeds. Typically earthy, but can be delicate and floral depending on how it's used.
Corn Flour: From whole kernels; not to be confused with cornstarch. Yields baked goods with sweet corn flavor and lovely pale gold color.
Xanthan Gum: Not a flour but a natural bacterial by-product. Often used with gluten-free flour to thicken and give structure.