Preheat the oven to 170°C. Grease the baking tin with butter and line it with baking paper, covering the bottom and the sides. Cut the remaining butter into small pieces.
If you have a microwave, place the chocolate buttons and butter in the microwave-safe bowl and zap for 2 minutes on high. Stir.
If the chocolate hasn't melted completely, return it to the microwave for 30 seconds on high and stir again. Repeat this last step if necessary, until melted. If you don't have a microwave, half-fill the saucepan with water. Place this over a medium-high heat and bring it to the boil, then turn the heat down until the water is simmering gently.
Place the chocolate and butter in the heatproof bowl and place this over the water, making sure that the water doesn't touch the bottom of the bowl. Stir until the chocolate and butter have melted.
Place the eggs in the large mixing bowl and whisk until well combined and a little frothy. Add the sugar and whisk in until the mixture is no longer grainy.
Pour the chocolate mixture into the egg mixture and stir to combine.
Scoop the avocado flesh into a separate small bowl and use a fork to mash it. Add the oil and stir to combine. Pour the avocado mixture into the chocolate mixture and stir to combine - there's no need to be too careful about this, just mix it more or less together.
Sift the flour and baking powder together into another clean bowl, and sift the cocoa. Add the flour and cocoa to the chocolate mixture in 3 or 4 batches, mixing to incorporate each time. Stir in a good pinch of salt, then fold in the raspberries and white chocolate, if using.
Pour the brownie batter into the lined tin and bake in the oven for 40 minutes. When ready, it should just be firm to the touch in the middle. Set aside in the tin to cool.
When fully cooled, cut into squares and serve. A few fresh raspberries served alongside look pretty.
Allow to cool for 1-2 hours.
Store in an airtight container for up to 3 days (if they last that long). The brownies also freeze really well, for up to 6 months. Defrost in the microwave for a warm fudgy brownie, or in the fridge if you're doing it in advance.
Tip: You could also make these with whatever chocolate you like, and add chopped nuts, raisins, popcorn, etc.