Measure the cereal into a large mixing bowl and set aside. Prepare a large gallon-sized ziplock bag or container with lid for coating.
In a microwave-safe bowl, melt the white chocolate in 30-second intervals, stirring between each, until completely smooth. This should take about 90 seconds total.
Quickly whisk the softened mascarpone, brewed espresso, and vanilla extract into the melted white chocolate until smooth and glossy. Work fast so the chocolate doesn't seize.
Pour the white chocolate mixture over the cereal and gently fold with a rubber spatula until every piece is evenly coated. Be patient and thorough to avoid breaking the cereal.
In a separate bowl, whisk together the powdered sugar, cocoa powder, and instant espresso powder until no lumps remain. Sift the mixture if needed for extra smoothness.
Transfer the coated cereal to the ziplock bag, add the powdered sugar mixture, seal tightly, and shake vigorously for 1-2 minutes until every piece is covered in the dusting.
Spread the coated cereal on parchment paper or a large baking sheet in a single layer. Let it sit at room temperature for 15 minutes to set completely.
Once set, break apart any large clusters and transfer to an airtight container. Shake off any excess powder if you prefer a lighter coating.
