Black and White Chocolate Streusel Cake

This was a recipe I sort of made up on a whim, but was really happy with how it turned out (it even elicited a love letter (to the cake!) from a friend who tried some. I was trying to get rid of a box of cake mix, but plan to try this again soon truly from scratch to see if I can get it even better.

1 package (18.25 ounces) yellow cake mix (butter recipe if possible)
1 package (3.4 ounces) vanilla instant pudding mix
1 1/4 cups milk
3/4 cup vegetable oil
3 large eggs

3/4 cups white chocolate chips
3/4 cups dark chocolate chunks/chips, both chopped into *very* small pieces (think almost flakes) in food processor

Streusel:
1/2 cup white sugar
6 tbsp all purpose flour
1/2 tsp salt
4 tbsp butter, melted

DIRECTIONS
1. Preheat oven to 350°F. Grease bundt pan.
2. Combine cake mix, pudding mix, milk, oil, and eggs in a large mixing bowl. Blend with an electric mixer on low speed for 30 seconds. After scraping sides of the bowl, increase mixer speed to medium and blend 1 1/2 to 2 min­utes more-- the batter will look thick but well combined. Gently stir in 1/2 cup of the white and dark chocolate flakes until uniformly mixed.
3. Make streusel topping, stirring in remaining cup of white/dark chocolate flakes.
4. Pour 1/3 of the batter into pan. Sprinkle ~1/3-1/2 of streusal mix on top, cover with 1/3 more batter, then more streusel. Spread remaining batter on top and sprinkle with remaining streusel.
5. Bake until cake is golden and fork comes out almost clean, about ~45-50min.
6. Place pan on a cooling rack for at least 15min.
7. To frost, you could make chocolate drizzle from chocolate chips, or feel free to use store-bought icing, put it in a ziploc bag, warm in microwave, cut of teeny-tiny opening at corner of bag and drizzle over cake in lots of thin lines. Enjoy!