2 cups (280 g) all purpose gluten free flour (I used Better Batter)
1 teaspoon xanthan gum (omit if your blend already contains it)
1 tablespoon baking powder
1/2 teaspoon kosher salt
1/4 teaspoon ground cinnamon
1/4 teaspoon freshly grated nutmeg
8 tablespoons (112 g) unsalted butter, room temperature
3/4 cup (150 g) granulated sugar
3 eggs (150 g, weighed out of shell) at room temperature, beaten
2 teaspoons pure vanilla extract
3/4 cup (6 fluid ounces) buttermilk, at room temperature
1/4 cup seedless jam or preserves
Coarse sugar, for sprinkling
Preheat your oven to 350°F. Grease or line a standard 12-cup muffin tin and set it aside.
In a large bowl, place the flour, xanthan gum, baking powder, salt, cinnamon, and nutmeg, and whisk to combine well. Set the bowl aside. In a large bowl, place the butter and sugar, and beat with a handheld mixer or in the bowl of a stand mixer with the paddle attachment until fluffy. Add the eggs and vanilla, and beat until smooth and lighter in color. The batter may appear a bit curdled, which is fine. Add the dry ingredients and the buttermilk to the butter mixture, alternating between the two and beginning and ending with the dry ingredients. Once you add the final dry ingredients, the mixture will be quite thick and a hand mixer may not be able to handle it. Just mix by hand until combined.
Fill prepared wells of muffin tin half full, then press a small hole in the batter with a moistened fingertip or dowel. Place 1 heaping teaspoon of jam in each hole, and then divide the remaining batter evenly among the wells, on top of the jam. Using wet fingers, spread the batter into an even layer and sprinkle each lightly with coarse sugar. Bake until mostly firm to the touch and a toothpick comes out with no more than a few moist crumbs attached (other than the jam, of course), about 22 minutes.
Allow the muffins to cool in the tin for about 5 minutes, and then transfer them to a wire rack to cool further. They will sink a bit as they cool. Serve them warm.