Step 1Using your hands, a large fork or a pastry cutter, mix the butter with the sugar and rice flour. The mixture will be moist and crumbly. Set aside.
Step 1Heat the oven to 375 degrees. Put 10 paper liners into a standard muffin pan.
Step 2In a large bowl, whisk together the rice, millet and oat flours, along with the salt and baking powder.
Step 3In a 2-cup measuring cup, combine the maple syrup, egg and enough milk for the combined ingredients to measure 1 cup, about one-fourth cup milk. Add the vanilla and gently whisk with a fork until fully combined.
Step 4Pour the wet ingredients into the large bowl with the dry ingredients, stirring with a wooden spoon or a whisk. Add the melted butter and stir just to combine, careful not to overmix. (Don't worry if there are small lumps.)
Step 5Fill the muffin cups with the batter and sprinkle 1 tablespoon Crumb Topping on each muffin. Bake until the muffins are set and a toothpick inserted comes out clean, about 14 minutes.