Place the cocoa powder and dark chocolate in a heatproof bowl. Pour the boiling water over the top and stir until the chocolate has completely melted. Let the mixture cool to room temperature, then stir in the sour cream until smooth.
In a separate bowl, whisk together the flour, baking soda, and salt.
In a third bowl, beat the butter, brown sugar, and vanilla extract until light and creamy. Add the eggs one at a time, mixing well after each addition.
Alternately add the flour mixture and the chocolate mixture to the butter mixture, mixing until you have a smooth batter.
Roughly chop the Oreo cookies and gently fold them into the batter.
Pour the batter into a greased and floured 25 cm (10-inch) Bundt pan.
Bake for 60 minutes at 350°F/180°C (conventional oven). Because this cake is so moist, a cake tester or skewer may come out with a few moist crumbs attached. That's exactly what you want.
Let the cake cool completely before making the topping.
Melt the white chocolate over a double boiler and pour it evenly over the cooled Bundt cake. Immediately sprinkle the chopped Oreo cookies over the still-soft chocolate.
You can slice the cake while the white chocolate is still slightly soft (which is absolutely delicious!), or let the chocolate set completely before serving. Either way, I'm sure you'll love this cake.