Lightly grease a baking sheet or line it with a silicone mat. Set aside.
Combine the sugar, corn syrup, and water in a heavy-bottomed saucepan. Stir gently over medium heat until the sugar dissolves.
Stop stirring once the mixture boils. Insert a candy thermometer.
Cook the syrup without stirring until it reaches 300 degrees Fahrenheit (hard crack stage). This takes about 15 to 20 minutes. Watch the temperature closely.
Remove the pan from the heat immediately when it reaches 300 degrees F.
Quickly stir in the baking soda, vanilla extract, and salt. The mixture will foam up rapidly.
Immediately fold in the mixed seeds until they are evenly coated with the hot syrup.
Pour the mixture onto the prepared baking sheet. Spread it thinly using a heat-safe spatula or the back of a spoon. Work quickly.
Let the seed brittle cool completely at room temperature until hard, about 30 minutes.
Break the cooled brittle into irregular pieces for your crunchy seed candy.
Notes
For a nut-free brittle, stick strictly to seeds.
If you want a vegan seed brittle, ensure your corn syrup does not contain animal products, though most commercial light corn syrups are vegan.
Store your finished brittle in an airtight container at room temperature for best texture. Avoid refrigeration, as moisture causes stickiness.
To get a crisp texture, do not stir the syrup after it starts boiling. Stirring causes crystallization, leading to a grainy, chewy result instead of a smooth brittle.