Brown Butter Pound Cake, Gluten-Free

Note: This recipe uses my gluten-free flour mix:
Jeanne's Gluten-Free All-Purpose Flour Mix (mix together and store in fridge):
1 1/4 C. brown rice flour
1 1/4 C. white rice flour
1 C. tapioca flour
1 C. sweet rice flour (also known as Mochiko)
2 scant tsp. xanthan gum

Special Equipment Needed
-loaf pan, 9"x2"x5.5"
-stand mixer is nice, but a hand mixer will do

2 1/4 sticks (18 TBL) unsalted butter
2 C Jeanne's Gluten-Free All-Purpose Flour Mix (sift before measuring)
1 tsp baking powder
1/2 tsp salt
1/2 C dark brown sugar
1/2 C granulated sugar
4 large or extra-large eggs
1/2 tsp vanilla extract
extra butter and tapioca flour for pan

Preheat oven to 325 degrees
-butter and flour pan with tapioca flour
-in a small bowl, mix together flour, baking powder and salt. Set aside
-in a large skillet, heat butter over medium heat until the milk solids on the bottom are dark chocolate brown.
transfer butter to a shallow bowl or baking pan and chill in freezer until just congealed--about 15 minutes. You want to get it to the consistency of soft but not melted butter
beat the butter for a couple of minutes--until fluffy
-add sugars, beat for a couple more minutes
-add eggs one at a time, beating after each addition
-add vanilla extract
-with mixer at slow speed, add flour mixture--do not over mix
-transfer batter to prepared baking pan
-thump on counter to release any trapped air bubbles
-bake at 325 degrees for about 1 hr 20 minutes or until a tester comes out clean
-remove from oven, cool in pan for about 10 minutes, then turn out onto a cooking rack to cool completely