Marbled Coffee Cake (Viennese), Gluten-Free

Marbled Coffee Cake

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

(you can also use the gluten-free flour mixture of your choice--just be sure it contains xanthan gum. Or, you can add 1/4-1/2 tsp. xanthan gum per cup of gluten-free flour. If you use bean flour, it will add a bean taste to the cake)

Special Equipment Needed
-a stand mixer is very helpful with this cake. As is a hand mixer--to beat the egg whites
-10 C bundt pan

1 1/2 C + 3 TBL Jeanne's Gluten-Free All-Purpose Flour Mix
1/2 tsp salt
2 oz semisweet chocolate (I use chocolate chips)
2 TBL rum
16 TBL (2 sticks) unsalted butter, softened
1/2 C confectioner's sugar
1-2 TBL lemon zest (I prefer to add 1 TBL)
1 TBL vanilla extract
5 large or extra large eggs, separated
1 C granulated sugar
extra butter and tapioca flour for pan

Preheat oven to 325 degrees
-butter and flour pan, set aside
-in a small bowl, mix together flour and salt; set aside
-in a double boiler, melt chocolate.  Add rum; set aside to cool slightly
-in the bowl of stand mixer with paddle attachment, beat butter
-add confectioner's sugar--continue to beat
-add lemon zest and vanilla--beat until mixture is pale and fluffy
-add egg yolks, one at a time, beating after each addition
-add flour/salt mixture to the butter mixture, mix until just combined (do not over mix)
-set this batter aside

-in another bowl, with a hand mixer, beat egg whites until frothy
-sprinkle with granulated sugar and continue to beat until stiff, glossy peaks are formed
-whisk 1/3ish of the egg white mixture into the cake batter to lighten it
-using a rubber spatula, fold in remaining egg whites to batter

-put chocolate/rum mixture into a medium bowl
-fold 1/3ish of the white cake batter into the chocolate/rum mixture to create your chocolate batter
-spoon 1/2 of the remaining white cake batter into your prepared bundt pan
-spoon entire bowl of chocolate batter on top of this
-spoon remaining white batter onto chocolate batter, smooth top to completely cover the chocolate middle layer
-stick a butter knife into the batter in the pan and swirl the batter around a bit to create the marbled effect. Smooth top
-thump pan on counter to release any trapped air bubbles
-bake at 325 degrees for about 1 hour or until tester comes out clean

-cool in pan for about 10 minutes, then unmold on to cooling rack and cool completely