Preparation Place a rack in center of oven; preheat to 325°. Lightly coat 20 molds of 2 brownie Bundt pans or the cups of a standard 12-cup muffin pan or 2 standard 6-cup muffin pans with nonstick spray; lightly dust with flour, tapping out excess. Whisk rice flour, baking powder, baking soda, kosher salt, and ½ cup (scant; 60 g) all-purpose flour in a medium bowl to combine. Beat butter in the bowl of a stand mixer fitted with the paddle attachment on medium speed just until smooth. Add granulated sugar and beat until pale and creamy, about 3 minutes. Scrape down sides of bowl with a rubber spatula. With motor running, add eggs one at a time, beating to incorporate and scraping down sides of bowl after each addition. Add vanilla and beat just to combine. (Mixture may look slightly curdled at this point, but don’t worry, it will come together in the end.) Using a fine-mesh sieve, sift in half of dry ingredients and beat on low speed just until a few dry patches remain. Scrape down sides of bowl and add sour cream and milk; beat just until incorporated. Sift in remaining dry ingredients and beat until combined (be careful not to overmix). Carefully divide batter among prepared pans (about 2 Tbsp. per mold for Bundt pans or about 3 Tbsp. per cup for muffin pan). Bake, rotating pans halfway through, until a tester inserted into the center of a cake comes out clean, 14–16 minutes for Bundts, 17–20 minutes for muffins. Let cakes cool in pan 10 minutes, then gently loosen cakes with a small offset spatula and turn out onto wire racks; let cool completely. Dip tops of cakes in glaze of choice; set back on racks, letting excess glaze drip down sides (for easy cleanup, lay a sheet of parchment paper underneath racks to catch any drips). If you want a more vibrant look, double-glaze cakes. Sprinkle cakes with toppings as desired (e.g., a pinch of sea salt on Milk Tea Glaze, coconut or sesame seeds on Matcha Glaze, and lemon zest on Strawberry Jam Glaze).Do ahead: Like doughnuts, gem cakes are best enjoyed the day they are baked but can be made 2 days ahead. Store airtight at room temperature.