There are many place in Hong Kong serving mapo tofu. But the most authentic one is from Tanyoto ( 譚魚頭 ). I don't mean to advertise for them, but their product tastes the same as that of Sichuan Restaurant's in Beijing. If you love spicy food, this is the place you must go. Mind you, they are going to paralyse your tongue.
麻婆豆付 Mapo Tofu HK$58 (US$7.4)
口水鷄Spicy Chicken HK$38 (US$4.9) Likewise, the spicy chicken here at Tanyoto is the most authentic. Tanyoto has three branches in Hong Kong, two in Beijing and two in Shanghai. You can also find them in 20 other cities in China.