Preheat oven to 350 degrees. Spray an 8-cup Bundt pan generously with baking spray and dust with granulated sugar, just as you would normally flour a cake pan. Turn out excess sugar from pan and discard. Set aside.
Whisk together the flour, baking powder, soda, and salt. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment, cream together the eggs, oil, and sugar until light and thick, about 5 minutes. Scrape down bowl before stirring in vanilla extract.
Working in three batches, add the dry ingredients, alternating with two additions of the milk. Scrape down the bowl as necessary to ensure the batter is uniform.
Before the final batch of dry ingredients are fully incorporated, stir in the instant pudding by hand. This ensures the batter will be completely uniform but not overmixed.
Divide ⅓ of the batter into a glass bowl and stir in the melted chocolate. Voila! Chocolate cake batter! (Note: this batter will be thicker than the plain pistachio.)
To marble the cake: Pour about 1 ½ cups of pistachio cake batter into the bottom of the prepared Bundt pan. Follow with about 1 cup of the chocolate cake batter dolloped throughout. Repeat the process of layering the pistachio batter and chocolate batter (always using more pistachio batter than chocolate) until all the batter is used. Gently swirl the batter in the pan with a chopstick or butterknife.
Bake cake in preheated oven for 45 – 50 minutes, or until a toothpick inserted into the center of the cake comes out clean.
Remove cake to a wire rack to cool upside down for 10 minutes before turning over and allowing to cool completely. (Cake should slide out of pan as it cools.)
When cake is completely cooled, drizzle with chocolate glaze and top with chopped pistachios.