Preheat the oven to 180°C and grease a 42x27x4cm baking tin with oil.
Toast almonds on a baking tray for 15 minutes until golden-brown, then cool.
Melt butter in a deep pan, add sugar, and stir until dissolved.
Boil the mixture until it reaches 120°C on a sugar thermometer.
Stir in three-quarters of the toasted almonds and the vanilla extract.
Boil slowly while stirring frequently until the temperature reaches 150°C.
Remove from heat immediately and pour the mixture into the prepared tin to cool for one hour.
Scatter chocolate over the warm toffee, let it melt, and spread with a palette knife.
Grind remaining almonds in a food processor and scatter over the chocolate.
Break the cooled toffee into chunks and store in the fridge.