Line a baking sheet with parchment paper.
Place the butter, sugar, salt, and vanilla in a heavy-bottomed small saucepan. Heat the mixture over medium heat until the butter and sugar dissolve.
Once the sugar dissolves, start whisking and continue to whisk as it cooks. If you stop stirring the butter and sugar will separate. Continue cooking for about 15-20 minutes. If using a candy thermometer the ideal temperature should be 285°C. If not using a thermometer the indicator of the toffee's readiness is the colour of the toffee. If after the mentioned time it is still pale in colour then you need to continue stirring until it reaches a deep golden brown colour.
Immediately pour the toffee over the prepared baking sheet and let it cool for 20 minutes.
Melt the milk chocolate in the microwave or over a double boiler. Once melted spread evenly on top of the toffee with a spatula.
Sprinkle on the toasted nuts and place the tray in the fridge until the chocolate hardens, about 1 hour.
Once all set, take the toffee out of the fridge and cut it into irregular pieces using a sharp kitchen knife. Store in an airtight container. Keeps well for 2 weeks.
