01 - Place roasted hazelnuts in a food processor and blend for 4 to 5 minutes until very smooth and creamy, scraping down the sides as needed.
02 - Melt the chocolate and butter together in a heatproof bowl over simmering water or in the microwave in 30-second intervals, stirring until smooth.
03 - Add the melted chocolate mixture, milk, powdered sugar, vanilla extract, and salt to the hazelnut butter, then blend until fully combined and smooth.
04 - Transfer the spread into a clean jar or airtight container and let it cool to room temperature; it will thicken as it cools.
05 - Store in the refrigerator for up to two weeks. Bring to room temperature before serving for optimal texture.