Preheat oven to 200 C. Line a 12 hole muffin tin with paper cases. Combine the flour, ground almonds, baking powder, xanthan gum and sugar in a medium sized bowl. In a separate bowl combine the milk, melted butter, eggs and vanilla extract. Gently stir the wet ingredients into the dry ingredients until just combined. Do not over mix. Spoon a tablespoon of mixture into each muffin case. Add a dollop of jam into the centre of each and then cover with another spoonful of mixture. Bake for about 20 minutes until well risen, golden brown and firm to the touch. Meanwhile prepare the topping. Melt the butter. Place the sugar in a wide shallow bowl. When the muffins are baked leave them to sit for about 5 minutes. Brush the tops with a little melted butter and then dip in the sugar. Transfer to a wire rack to cool. Serve warm.
My mind is buzzing with the number of variations you could try as fillings for these. I would try substituting the jam with a home made lemon curd or with a dollop of creme patisserie for the ultimate jam doughnut muffin.