Chestnut Cupcakes with Chocolate Frosting (GF)

I picked up a jar of sweetened chestnut purree at an import store. As I was looking on the internet for muffin recipes I came across Tartelette's Chestnut Cupcakes. I actually HAD all of the ingredients so I couldn't resist...
Substitutions marked below:
1 cup all-purpose flour 4 oz brown rice flour mix + .5 oz coconut flour
1/2 teaspoon baking soda
1 stick unsalted butter, softened Used 1/2 stick
1/2 cup granulated sugar 1/3 c
1/2 cup firmly packed light brown sugar 1/3 c
2 large eggs, at room temperature
3 ounces chestnut puree
1/2 cup buttermilk
1/2 teaspoon vanilla extract
1/2 tsp guar or xanthan gum

I followed the recipe instruction exactly as written. I was too lazy to make piped frosting so I just melted the butter, cream cheese, and chocolate together in the microwave and stirred until smooth.

Oh WOW. I just finished eating one and they are out-of-this-world good. And decadent. Ben's review... "Oh. It's really good. You get a bite of chocolate and a little bit of nut. It is SO good."