Preheat the oven to 350ºF. Line an 8-inch square pan with parchment paper and spray with nonstick cooking spray.

In a medium saucepan, melt the butter over medium-low heat. As the butter melts, it’ll start to foam, then turn a clear golden color. Continue to stir the butter, scraping up any browned specks from the bottom of the pan as it starts to brown. It should have a nutty aroma.

Once browned and nutty, turn off the heat and allow to cool for 5 minutes. Add espresso powder to the hot water and stir until dissolved.

Stir the brownie mix, espresso water, egg(s), and vanilla into the browned butter until just combined. Fold in the crushed Oreos and toffee bits. Pour batter into prepared pan and bake for 30-32 minutes**, or until a toothpick inserted in the center comes out with few crumbs attached. Allow to cool completely.

Put the Oreos in a large ziplock bag, seal, then pound with a rolling pin until finely crushed. Pour into a large bowl and stir in the cream cheese until mixture is smooth and thoroughly  incorporated. Pour on top of cooled brownies, and spread evenly.

In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, marshmallow fluff, vanilla, and salt together until smooth and creamy. Scrape down the sides of the bowl, then add the powdered sugar. Mix on low until combined. Add the crushed Oreos and continue to beat. Spread frosting evenly over truffle layer. Top with extra Oreos and sprinkles, if desired.