Preheat your oven to 180°c/160°c fan and line your baking tin with greaseproof paper.
Measure out your peanut butter, unsalted butter and dark chocolate into a bowl and melt gently in a bain marie. Alternatively, you can melt it carefully using the microwave on low power in 40 second increments until the chocolate is melted. Mix until uniform in colour and texture, and put aside to cool slightly.
Mix your eggs and sugar with your hand mixer, or use a stand mixer with the whisk attachment, on high. You want the mixture to roughly double in volume and be very pale and fluffy. This could take 3-8 minutes depending on your mixer's speed.
Once the egg and sugar mix is voluminous enough, gently fold in the chocolate/butter/peanut butter, plus your vanilla paste or extract. Use figure-of-eight motions, taking care not to knock out too much air, until fairly well combined. Don't worry about making it completely uniform for now.
Whisk your ground nuts and seeds, flour (if using), brewer's yeast and cocoa powder together in a bowl briskly until all lumps are gone and the ingredients are evenly dispersed. Add in your white and milk chocolate, chopped into chunks (I like a variety of sizes, just for texture.) Fold the dry ingredients into the wet mixture gently until no dry streaks remain.
Pour your brownie batter into the lined baking tin, spreading evenly if necessary.
Bake for 25-35 minutes, depending on how you like your brownies. Slightly longer will yield a more fudgy/chewy brownie, while less cooking time will give you more of a dessert-style gooey brownie. To check that your brownies are cooked, give your pan a little shake. If any of the batter is still wobbly it needs a little longer.
As difficult as it may be, leave your brownies to cool completely before slicing. This will give you much tidier pieces. Make 6 slices in each direction to yield 36 squares. This is the serving size I'd technically recommend in one go but let your heart guide you. I'd be lying if I said I only ever had one at a time.
Serve with tea or coffee, or with a scoop of ice cream if you're feeling decadent (as you should!)