For the gluten-free flour blend: In a large bowl, whisk together the rice flour, tapioca flour, potato starch, xanthan gum and salt.
For the pecan crumble: In a small bowl, stir together the pecans, brown sugar and cinnamon with 2 teaspoons gluten-free flour blend.
For the cake: Preheat the oven to 350 degrees F and grease a 12-cup bundt cake pan with butter or gluten-free cooking spray. In a medium bowl, whisk together the baking powder, baking soda and salt with 1 1/2 cups gluten-free flour blend.
Using an electric mixer, in a large bowl, cream together the butter and brown sugar on medium-high speed until fluffy. Beat in the eggs one at a time. Add the flour mixture and buttermilk to the egg mixture, alternating and beginning and ending with the flour mixture. Transfer half of the batter to the prepared pan and sprinkle half of the pecan crumble mixture on top. Repeat with the remaining batter and pecan crumble mixture. Using a butter knife, swirl the crumble mixture into the batter in a crisscross motion. Bake until a toothpick inserted in the center comes out clean, 45 to 50 minutes. Let cool completely on a wire rack and invert onto a cake plate.
For the frosting: In a small saucepan, bring the brown sugar, butter and milk to a boil over medium heat. Let boil 1 minute, whisking constantly. Remove from the heat and stir in the vanilla. Gradually whisk in the confectioners' sugar until combined. Whisk for about 3 minutes to cool slightly; pour the frosting over the cooled cake and top immediately with the pecans to decorate.