Vegan Gluten-Free Chinese Five Spice Cupcake with Crystal Ginger Frosting

This is my first attempt baking gluten-free cupcakes. The dough turned out to be a little thick, but the cake came out of the oven beautifully. The only thing I'm concerned; the unsmooth top! I wonder, should the liquid need to be more, or is it because of the buckwheat flour again? I have baked gluten-free cakes (without buckwheat flour) before and the dough was usually more watery. I will have to readjust this for the future, but anyway, the cake is fluffy and moist, with a little chewy feeling. It is not very sweet and so the sugar could be 1/2 cup or more, instead of 1/3 cup.

*The sponge became much harder the next day. So, this is something to be adjusted for the next time, I don't know how yet...

Cupcake (makes 12):
-dry
3/4 cup rice flour
1/4 cup tapioca flour
1/4 cup buckwheat flour
2 Tbsp Arrowroot flour
2 tsp xanthan gum
1/3 cup coco
1/4 tsp salt
1 tsp baking powder
1 tsp baking soda
2 tsp Chinese Five Spice
-wet
1/3 cup sugar
100 ml olive oil (a little less than 1/2 cup)
1 tsp vanilla essence
1Tbsp vinegar + rice milk = 1 1/4 cup

Direction:
preheat the oven at 350 degree.
Place papers.
1. Mix vinegar and milk, leave it aside.
2. Measure all the dry ingredients, mix well, leave it aside.
3. In a large bowl, combine sugar, oil, and vanilla. Beat well with an electric mixer.
4. Slowly introduce the milk mixture into "3". This will be very watery, but keep beating.
5. Add the flour mixture at once, beat well. There is no gluten so no worry for over-beating. The batter will be smooth.
6. Divide the batter into 12, filling about 2/3 each cup. Bake for about 20 minutes.
Frosting:
1 cup powdered sugar
2 Tbsp shortening-room temperature
2 Tbsp margarine-room temperature
roughly 6 crystal ginger (leave 2 for the topping-each cut in 6)
a few drops vanilla essence
1 to 2 Tbsp milk

Direction:
1. Mix shortening and margarine with a fork, add the powdered sugar.
2. Add vanilla. Mix well while introducing a little bit of milk at a time.
3. When it reaches at a nice creamy texture, mix in the chopped crystal ginger.
4. Decorate the cakes when cooled.