Preheat oven to 350°F/175°C and line an 8×8 inch baking dish with parchment paper.
Place butter in a microwave safe bowl and microwave for 30-45 seconds, until butter is melted and hot. Add 3/4 cup white chips and let sit for 1 minute. Stir until smooth, if needed place back in microwave for an additional 15-30 seconds to melt through.
Stir flour, sugar, white chocolate chips, baking powder and salt into the melted chocolate mixture. Stir in eggs and lemon curd and poppy seeds until well combined. Transfer to prepared baking dish, spreading evenly. Bake for 30-35 minutes, until cooked through.
Let cool completely then cut into squares. Dust with powdered sugar and serve at room temperature or chilled.