Preheat oven to 350 degrees F. Generously spray a 10 inch bundt pan with non-stick cooking spray.
Stir together flour, baking powder, baking soda, and salt in a medium bowl. Set aside.
In a large bowl, beat butter until lightened in color and creamed. Add sugar and beat until fluffy. Add eggs, one at a time, and beat until well combined. Add vanilla and beat until combined.
To the creamed mixture, alternately add ⅓ of reserved flour mixture, then ½ of buttermilk, then ⅓ of reserved flour mixture, then ½ of buttermilk, then final ⅓ of flour mixture, beating between each addition until just barely combined. Take out 2 cups of batter and reserve in a small bowl. Dump remaining batter into prepared pan.
Gently melt ⅔ cup chocolate chips (can melt in microwave gradually, cooking for about 15 seconds at a time, stirring in between, until melted). Stir melted chocolate into the reserved 2 cups of batter, stirring until combined. Dump chocolate batter onto vanilla batter in pan. Run a knife through the mixture until swirled slightly.
Bake in preheated oven for 45-50 minutes, or until cake is set and toothpick inserted near the center of the cake comes out clean. Cool about 15 minutes, then remove cake from pan by inverting onto a plate.
For the Glaze:
While the cake cools, heat together all the glaze ingredients, whisking frequently, until smooth. Drizzle over cooled cake.
Recipe by Wright Family Table at https://www.wrightfamilytable.com/milk-chocolate-and-vanilla-marble-cake/