To make the flax egg, combine flaxmeal + hot water, set aside for 1–2 minutes.
Beat vegan butter + brown sugar until creamy. Add flax egg + vanilla extract, mix until combined.
Add flour, cornflour, chai spice, baking soda, baking powder; fold in 3/4 of the chopped chocolate.
Roll 1½ tbsp dough into balls.Press indent in dough, insert frozen Nutella, wrap dough around filling. Press the remaining 1/4 of the chocolate onto the tops of the cookie dough balls. Refrigerate for 3 hours–overnight.
Preheat oven to 375°F (170°C) and line a baking sheet with parchment.
Place 6 cookies per sheet. Optional: Dollop ¼–½ tsp Nutella on top. Bake 11–13 minutes. Tap sheet to flatten.
Cool 5 min on sheet, 10 min on rack; sprinkle with sea salt and serve warm.