Preheat oven to 180C, and butter and line a 22cm-square cake tin with baking paper, allowing paper to overhang sides. Spread almonds on a baking tray and roast until golden brown and fragrant (6-7 minutes). Cool, then coarsely chop.
Melt butter and dark chocolate in a bowl over a saucepan of simmering water, stirring occasionally, until smooth (2-3 minutes). Remove from heat, add sugar, then mix in eggs one at a time. Sieve flour, cocoa and baking powder over mixture, add malt powder and a pinch of salt and stir to combine (don’t overwork the dough or the brownie will become cakey), then stir in almonds, milk chocolate and cacao nibs, reserving a little of each to scatter on top.
Spread batter in prepared tin, scatter with reserved almonds, chocolate and cacao nibs and bake until set around the edges but still a little fudgy in the centre (45-50 minutes; a skewer inserted should withdraw with just a little mixture on it). Cool completely, then cut into pieces and store in an airtight container for up to 5 days.