Lightly grease a baking tray with sunflower oil or line with a non-stick liner. Toast the almonds in the oven for 8- 10 minutes until fragrant and allow to cool before oarsely chopping. Set them aside.
Have the bicarbonate of soda and vanilla extract measured out separately and ready to go.
Place the water, butter, caster sugar and brown sugar in a small saucepan. Bring the mixture slowly to the boil over a medium heat, swirling the pan very gently to ensure all the sugar dissolves before it starts to boil. Place a candy thermometer in the saucepan and boil the mixture until it reaches 300 F (150 C) which is known as hard crack. As soon as it reaches the correct temperature take the pan off the heat and add the bicarbonate and the vanilla extract. Mix to combine and then pour the hot mixture onto the prepared tray. Leave to set for a few minutes and then sprinkle the chocolate chips on top. Leave them to melt for a few minutes and then using a spatula or a palette knife spread the chocolate out over the toffee in an even layer. Add the chopped nuts if you are using them and then allow the toffee to cool for about 15 minutes. You can place it in the fridge to speed up the process a bit or leave it somewhere cool. Once set you can break the toffee into chunks and serve.