Preheat the oven to 350°C / 180°F. Lightly grease the base and sides of a 25cm x 10cm loaf tin and line the base with greaseproof paper.
In an electric spice grinder, whiz the seeds into a flour. Transfer to a bowl, add the cassava flour, baking powder and salt and mix to combine.
Break the eggs into a jug, add the water and beat lightly to blend.
Cut the butter into chunks, place into a food processor and whiz on low speed until creamed.
With the motor running add 1/3 of the egg, then 1/3 of the flour. Continue in lots, mixing well after each addition.
Spoon the batter into the tin. Shake to distribute evenly and smooth the top with a rubber spatula dipped into water. If using, sprinkle over the extra seeds and press lightly to embed them into the surface.
Bake for 1 hour. Allow to rest for 30 minutes before turning out.
Cut into slices, wrap, and store in the freezer.