Preheat the oven to 350°F (175°C) and prepare a 7-inch (18-cm) square ceramic baking dish. Grease the bottom and sides, and line it with parchment paper.
Sift the flour, cocoa powder, tapioca starch, salt and baking powder into a medium-sized mixing bowl and mix, using a whisk to evenly distribute the ingredients.
In a small bowl, whisk the sugar with the hot water to dissolve it and leave to cool. Once the sugar and water have cooled down to warm but not hot, make a well in the dry ingredients and add the sugar-water mixture, oil and lemon juice. Fold gently to combine, then whisk to get the batter smooth. Add the chocolate chips, reserving a few pieces to decorate the brownies.
Transfer the batter into the prepared baking dish, gently shake to level and smooth the top. Scatter the reserved chocolate on top and bake for 20 minutes. When the brownies are done, the center will still seem a little undone and wobbly, but it will set in the pan while it cools down.
Let the brownies cool down completely before slicing. Store the brownies in an airtight container and refrigerate for up to 10 days.
If you serve them directly from the refrigerator, they will be firm and resemble a chocolate bar; when served at room temperature, they will be incredibly fudgy and soft.