Line a baking tray with baking paper and oil the paper.
Take a large heavy based saucepan then add the sugar, water, agave syrup and butter cubes. Mix with a wooden spoon until the sugar is completely moistened then cook over medium high heat. The sugar will completely dissolve and form a clear syrup then start to foam, then thicken and turn golden. Watch carefully and stir occasionally.
When the mixture is golden brown (or a sugar thermometer reaches 150 degrees C); after about 15 minutes of cooking remove from heat immediately, add the baking soda and salt then stir well to mix. The mixture will foam, which is normal.
Then add the pumpkin seeds and mix well to distribute in the toffee. Be careful not to splash the hot toffee.
Without waiting, pour the toffee on prepared baking tray and spread with the back of a wooden spoon to even the surface. Oiling the spoon so that it will not stick to the toffee.
Allow the toffee to cool completely before breaking into large irregular pieces. Keep brittle in an airtight container for up to two weeks.