Pre-heat your oven to 160C Fan/180C/350F/Gas Mark 4, and line a few baking trays (or however many you own) with baking paper
Add the plain flour, baking powder, salt, chia seeds and sesame seeds into a large mixing bowl
Stir the mixture together with a wooden spoon so the seeds are evenly spread out
Add the butter and rub the mixture together with your hands until it resembles breadcrumbs
Make a well in the middle and pour in the cold water
Mix together until a rough dough is formed. You may need to add a little more water if the mixture is too dry
Do not over work the dough, just shape it into a ball, cover with cling film and chill for 20 minutes in the fridge
Roll the dough out between two sheets of baking paper until it is 1-2mm thick
Use a 6cm round cutter to cut out the cracker shapes, or you can use any size or shape of cutter you like
Place onto the baking trays and prick the crackers a few times each with a fork. Sprinkle the coarse sea salt over them
Bake for 10-15 minutes until they go golden around the edges. Cool on a wire cooling rack
Store in an airtight container and eat within 5 days. Or freeze for up to 8 weeks