First, melt the chocolate and butter together in a double boiler. Once both are melted, take them off the heat and stir with a spatula until the mixture is completely smooth, with no lumps remaining.
In a separate bowl, sift together the flour, baking powder, salt, and coffee powder. Back in the melted chocolate mixture, beat in the caster sugar, vanilla essence, and chocolate extract for 3 to 4 minutes on low speed until well combined.
Gently fold in the sifted dry ingredients, walnuts, and milk until everything is just moistened and combined. Be careful not to overmix the batter. While you're doing this, preheat your oven to 180 °C. Grease an 8-inch square tin and line it with parchment paper, leaving an overhang on two sides to help lift the brownies later.