Melt white chocolate in short microwave bursts until smooth. Add butter and stir until fully melted and emulsified. Whisk in sugar and eggs until glossy. Stir in pistachio paste and salt. Fold in flour and baking powder just until incorporated.

Transfer batter to the pan and smooth the top. Bake until edges are set and center is slightly soft, 18 to 22 minutes. Cool completely, then refrigerate for 1 hour.

Combine white chocolate and heavy cream in a heatproof bowl. Microwave in short bursts, stirring until smooth. Spread over chilled brownies and refrigerate until set, about 3 hours.

Finish with chopped pistachios and a drizzle of pistachio paste. Lift out using parchment and cut into squares with a warm knife.