Preheat the oven to 350 degrees F and line an 8x8 inch metal baking pan with parchment paper. Alternatively, lightly grease the pan.
In a large microwave-safe bowl, add the vegan butter, 1 cup vegan chocolate chips and unsweetened soy milk. Microwave for 1-2 minutes, stopping every 30 seconds to whisk well, until melted and smooth and the mixture starts to simmer.
Add the sugar immediately and whisk constantly for about a minute until it dissolves into the warm mixture.
Whisk in the vanilla and unsweetened cocoa powder until well combined.
Add the flour, baking powder and salt and stir to combine. Fold in the vegan white chocolate chips (or regular chocolate chips), if using.
Scoop about a third of the batter into the prepared pan and press it down evenly. Layer about 16 whole cookies over the batter evenly. Pour the remaining brownie batter over the cookies, and spread it evenly. Chop the other cookies coarsely and put them on top, pressing gently down into the batter.
Bake in the center rack of the oven for 25-30 minutes.
Let the brownies cool in the pan for at least 30 minutes before pulling them out using the parchment paper. Allow them to cool for another 30 minutes or so if possible, for cleaner sliced brownies.