Jinjiang Tea House is one of my favorite Hakka restaurants in Taipei. Everything is delicious and decently priced.
The shop sells lei cha (擂茶), literally pounded tea, a traditional Hakka "tea" drink made by grounding or pounding several different types of nuts, grains, and seeds together. The tea, which is commonly green in color, is in powder form and made by adding water. Jinjang sells many different flavors of leicha from traditional without sugar to almond and chocolate.
Foodwise, everthing here is great. For something different try the betel nut flower (檳榔花)--one of my favorites.