Position a rack in lower third of oven and preheat to 375°. Line 12 muffin cups with paper liners.
In a large bowl, whisk brown rice flour, baking soda, xanthan gum, cinnamon, ground ginger, allspice, and salt together until well blended. Set aside.
Peel fresh ginger. "I like to use a spoon because it gets around the curves better than a peeler. I peel toward me for the most control." Slice ginger very thinly across the grain until you have 1/4 cup (30 g). Put ginger slices and brown sugar in a food processor and pulse into a purée.
Cut butter into cubes and melt it in microwave. "Cubing it helps the butter melt evenly and helps keep it from spitting as it heats." Add hot butter, flour mixture, molasses, and egg to food processor with ginger-sugar purée and whirl 20 seconds. "Process any longer, and the xanthan gum could lose its thickening power." Scrape bowl, add buckwheat flour and 1/2 cup hot water, then process exactly 5 seconds more.
Pour batter into a liquid measuring cup, then divide it among lined muffin cups, wiping your finger across the rim of the measuring cup to cut off the flow of batter without dripping all over the pan. The cups should be 3/4 full, using every bit of batter.
Bake until a toothpick inserted in the center of a muffin comes out with a few dry crumbs, 15 to 20 minutes. Set muffins on a rack for a few minutes to firm up, then carefully remove from pan (tilting it helps) and set on rack to cool completely.
*Find xanthan gum at bobsredmill.com.
Make ahead: Up to 4 days at room temperature in an airtight container; up to 3 months, frozen airtight. Bring to room temperature before serving.
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.