From the Wall Street Journal, http://online.wsj.com/news/articles/SB10001424127887324009304579045414133137186

[Notes: We substituted strongly flavored Ginger Beer for the Root Beer
on our second batch and it was awesome.  We also used slightly less
granulated sugar, 1 cup instead of 1.25.  We used salted butter and
omitted the salt.  We also used half whole wheat pastry flour.  I
added 1 tsp powdered ginger.

I tried substituting vegetable oil instead of butter, and it worked
fine.  I used only 1/2 tsp salt.  Recipes always call for too much
salt.  You don't need to heat it.  It's more fun that way because the
soda bubbles around in the sugar and flour (and baking soda!)]

			 Root Beer Bundt Cake

Make this cake the night before serving to let the flavor
intensify. For a more pronounced root beer taste, swap out 1/2 cup of
root beer for root beer schnapps. And whatever you do, do not use a
diet root beer.

Total Time: 1.25 hours Makes: One 10-inch Bundt cake

Root Beer Bundt Cake
Gregory Reid for The Wall Street Journal, Food Styling by Heather Meldrom

For the cake:

1 stick unsalted butter, cut into 1-inch cubes
2 cups flour, plus more for dusting pan
2 cups root beer, preferably a natural brand such as Sprecher or Boylan
1 cup dark cocoa powder
1.25 cups sugar
1/2 cup packed dark brown sugar
1.25 teaspoons baking soda
1 teaspoon salt
2 large eggs

What to Do

1. Preheat oven to 325 degrees. Butter and flour a 10-inch Bundt pan.

2. In a small saucepan over medium heat, heat butter, root beer and
cocoa powder until butter is completely melted. Add sugars and whisk
until dissolved. Remove mixture from heat and set aside to cool.

3. In a large mixing bowl, whisk together flour, baking soda and salt.

4. In a small bowl, whisk eggs until just beaten, then whisk into
cooled cocoa mixture until combined.  [Note:  To avoid the "chocolate
scrambled eggs with sand" effect, if chocolate mixture is not below
120 degrees or so, you should put a few tablespoons at a time of
chocolate in with the eggs while whisking, until you've got quite a
lot mixed in that way, and then whisk the chocolate-egg mixture into
the chocolate mixture.]  Gently fold flour mixture into chocolate
mixture. Do not overbeat. Mixture will be slightly lumpy.

5. Pour mixture into Bundt pan and bake, rotating pan halfway through
baking, until a toothpick inserted into cake comes out clean, 35-45
minutes.

6. Remove from oven and let cake cool in pan. Once completely cool,
use a knife to gently loosen sides of cake from pan and invert onto a
cooling rack.

For the frosting: [Note: we didn't frost ours; it was moist enough.]

2 ounces 60% chocolate, melted and slightly cooled
1/2 cup unsalted butter, at room temperature
1 teaspoon salt
1/4 cup natural root beer
2/3 cup dark cocoa powder
2.5 cups confectioners' sugar

7. Make frosting: Place all ingredients in a food processor and pulse
in short bursts until frosting is shiny and smooth. If necessary, thin
with 1-2 tablespoons whole milk, pulsing to combine.

8. Use a spatula to spread frosting in a thick layer over top of
cake. Let frosting set before serving.

--Adapted from Matt Lewis and Renato Poliafito, Baked, Brooklyn, N.Y.