Mocha Marbled Coffee Cake

I made the following cake for my sister-in-law's baby shower. I made it once a couple of years ago but this time, implemented a couple methods from America's Test Kitchen, which they use in their chocolate bundt cake recipe. If interested, I could email you the recipe in Word.

Mocha Marbled Coffee Cake
Makes 16 servings

2 oz bittersweet or semisweet chocolate, chopped
1/3 cup cocoa powder
1/3 cup boiling water

1 Tbsp butter, melted
1 Tbsp cocoa powder

12 Tbsp butter (1 ½ sticks), softened
1 ½ cups granulated sugar
3 whole eggs
1 ½ tsp vanilla extract
3 Tbsp instant coffee powder or 2 Tbsp espresso powder, dissolved in 2 Tbsp hot water
2 ½ cups flour
1 ½ tsp baking powder
¾ tsp baking soda
¼ tsp salt
1 ¼ cups sour cream (regular or light)

1. Preheat oven to 350 degrees.
2. Prepare chocolate mixture by pouring boiling water over cocoa and chocolate, whisking gently to melt and combine. Whisk until smooth and shiny. Set aside.
3. Prepare bundt pan by combining melted butter and cocoa to form a smooth paste with a fork or small whisk. Using a pastry brush, coat bundt pan with mixture, making sure to get in all crevices and far enough up the sides. Set aside.
4. Using a mixer, combine butter and sugar and beat until light and fluffy. Beat in the eggs, one at a time, mixing until combined. Add vanilla and dissolved coffee.
5. In a separate bowl, combine flour and next three ingredients (through salt).
6. Add dry ingredients to the egg mixture, alternating with the sour cream. Mix just until combined for each step. Scrape down sides and bottom.
7. Put half the batter in a separate bowl and add the chocolate mixture, stirring to combine.
8. Place 1/4 cup of the coffee batter in two opposite sides of the pan. Place ¼ cup chocolate batter in the other two parts; spread until they meet. Continue placing about ¼ cup of each batter on top of the other kind of batter, to create a checkerboard pattern. Spread to even out after the last of the batter is done.
9. Bake for 35-45 minutes or until a toothpick inserted into the middle comes out with scant crumbs on it (but not doughy). Baking times may vary depending on your oven.
10. Let cool on a wire rack for 10-15 minutes; turn out of pan. After the cake is completely cooled, dust with powdered sugar, if desired, or garnish each serving with whipped cream.