Start by making your flax eggs. You'll need either two eggs or two flax eggs, which are super quick and easy to make. Mix in a small bowl 2 tablespoons of ground flaxseeds (1 per "egg") with anywhere from 6-9 tablespoons of water. After 6 tbs, add one spoon at a time until you have a somewhat firm yet runny consistency.
Let the flax eggs sit for about 10 minutes while you prep the other ingredients.
Preheat oven to 350 degrees.In a large bowl, mix flours, salt and baking powder together (1 teaspoon baking powder per cup of flour, which is 8 teaspoons or roughly 2.5 tablespoons). Then, mix in your bran flakes, nuts and raisins. In a large glass bowl, melt coconut oil. You can microwave it, or heat separately in a small saucepan then pour it into a bowl. While the oil is warm, mix in the sugar to dissolve it a bit. When the oil and sugar is mixed well, whisk in flax eggs. Finish by adding yogurt and vanilla.
Make sure the wet ingredients are mixed together very well, then add them to dry ingredients. Mix until just combined (don’t over-mix at this point).
Pour batter into a casserole dish lined with parchment paper. Bake for 1 hour until a toothpick comes out clean.
Cool the rusk bread/cake for 10 minutes in the dish, then transfer to a wire cooling rack until completely cool (about 20 minutes). If you cut them while they're still hot, they will fall apart.
Cut rusk size pieces—basically rectangle cubes you can comfortably hold and dip into tea (about 1-2" x 1-2" x 3-4"). Go with your preference here.
Now the rusks need to dry. Drying times will vary depending on your method. If you're using an oven, try about 6 hours at 180 degrees. If you're using an air fryer, try 250 degrees for only 20 minutes. Every oven is a little bit different, so you'll have to do some estimating yourself and check that your rusks aren't burning every so often. If you're drying them in the oven, maybe set an alarm for every hour to check on them.
Once dry, store at room temperature in an airtight container for up to a few months.