Line your square pan with parchment paper, leaving overhang for easy removal.
Combine the chocolate chips and butter in a microwave-safe bowl. Microwave in 30-second bursts, stirring each time, until the mixture is glossy and smooth, about 1-2 minutes.
Add the sweetened condensed milk and vanilla extract to the melted chocolate mixture. Stir well until fully combined, creating a thick, shiny chocolate paste.
Pour the mixture into the prepared pan, then use a spatula to spread it evenly across the surface. Press down gently to ensure it’s compact and smooth on top.
Let the mixture sit at room temperature for about 15 minutes to slightly relax, then transfer to the fridge. Chill for around 2 hours until the mixture is firm and glossy.
Once set, lift out the block using the parchment overhang. Use a sharp knife to cut into 16 even squares, pressing gently and wiping the knife between slices for clean edges.
Serve immediately, or store in an airtight container in the fridge for up to a week. For a decorative touch, drizzle with melted chocolate and let it set before serving.