Preheat the oven to 350 F (180 C)
Line a 9-inch (23 cm) square baking pan with parchment or foil and lightly oil the bottom.
In microwave, melt the dark chocolate, butter and 1 tbsp (15 mL) of the espresso powder in a large (microwave safe) measuring cup. Stir together to combine. Set aside.
In bowl, beat the eggs on medium high until frothy. Add the brown sugar and beat until the mixture is smooth. Add the melted chocolate mixture into the egg-sugar mixture a little at a time. Incorporate it slowly and beat well for a good minute. The chocolate will look smooth and glossy.
In a food processor, combine the brown rice, rice flour, espresso powder, salt and baking soda until well combined. Add the rice mixture into the chocolate mixture and beat well for a minute. Add the vanilla, beat one minute.
Fold in the nuts and spread the batter into the prepared baking pan, smoothing the top to even it out. Sprinkle with chocolate chips and remaining nuts. Shake the pan a little to help even out the batter.
Bake in the centre of the oven just until the brownies are set, about 30 to 35 minutes. Don't overcook (they should be fudgy rather than dry and crumbly)
Cool completely on a wire rack before covering and transferring to refrigerator to chill for at least two hours and up to one day before cutting into squares (they are easier to cut cold).