Make brownies: Preheat oven to 350ºF. Butter an 8-inch square pan. Line bottom and sides with parchment; butter paper. Dust paper with cocoa, tapping out excess.
In a microwave-safe bowl, microwave butter and chocolate chips on low for 20-second intervals, stirring after each, until melted and combined, about 1 minute total. Whisk in sugar (mixture will look grainy). Whisk in eggs and vanilla, mixing until smooth.
In a separate bowl whisk together cocoa, flour, baking powder and salt. Stir flour mixture into chocolate mixture until just combined. Fold in half of peppermint bark. Pour mixture into prepared pan; smooth top. Bake, rotating pan halfway through, until a toothpick inserted in center comes out with a few crumbs, 25 to 30 minutes. Let cool on a wire rack for 30 minutes. Run a sharp knife around edge of pan, then transfer brownies to rack to cool completely.
Make glaze: Place brownies on a cooling rack set over a rimmed baking sheet. Warm cream in a small pot over medium-high heat, stirring occasionally, until steaming, 2 to 3 minutes. Remove from heat and add chocolate chips. Let sit for 3 minutes, then whisk until smooth. Pour over brownies. Scatter top with remaining peppermint bark. Before serving, let stand--or refrigerate--until glaze sets.
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