Preheat oven to 375 degrees (F) and grease a 12-cup muffin tin or line with cupcake papers.
Place the gluten-free flour, baking powder, baking soda, and salt in a large bowl and whisk until combined.
Add sugar, lemon zest, lemon juice, sour cream, butter, eggs, and vanilla to a mixing bowl and beat until well combined, 2 - 3 minutes.
Add the dry ingredients to the mixing bowl and mix on low just until combined. Be sure to not over-mix your batter, or you'll have rubbery muffins. The batter will be slightly stiff.
Fill each muffin cup halfway with the batter. Make a little well in the center with the spoon. Add 1 tablespoon of lemon curd into each muffin hole in the well you made. Top with the remaining batter to cover the lemon curd. Smooth out the tops if you want rounded muffins out of the oven. Mine didn't change shape at all as they baked.
Bake 15 - 20 minutes until lightly golden brown and a toothpick inserted in the center comes out mostly clean.
Leave to cool in the pan for at least 10 minutes. I brushed the top of the warm muffins with a pastry brush dipped in the remaining lemon curd, but that's optional.
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