Make the dough for the wrappers by sifting the flour, salt and baking powder into a large bowl. Slowly add the water, 100ml to begin with and then tablespoon by tablespoon until you have a medium-stiff dough.
Knead the dough lightly in the bowl and then coat with the oil and allow to rest for 30 mins, covered with a tea towel.
To make the sauce, roast the tomatoes in the pre-heated oven for 20 mins. Once roasted, remove the skins from the tomatoes. Blend them with the chillies in a mixer, along with the coriander, vinegar, salt, sugar and sichuan peppercorns. Blend until smooth and, with the mixer running, slowly drizzle in the oil until you have a smooth sauce. Leave to cool. To make the prawn filling, heat the oil in a pan over a medium heat and fry all the spices and salt for a min until fragrant. Add the butter, ginger, garlic, chilli and prawns and cook until the prawns are cooked. Set aside to cool.
Divide the dough into quarters and working one-quarter at a time, roll out until very thin (2 - 3mm thick)Using an 8cm cutter, cut out rounds from the sheet of dough. Remove the excess dough.
Place 1 tsp of filling in the centre of the dough and pleat the dough circle, pinching the top together as you go. Repeat with the remaining wrappers and filling until everything is used up.
Bring a frying pan of water to the boil and place a bamboo steamer over the top. Place some momos in the steamer 1cm apart and steam for 15 minutes, or until the wrappers turn a glossy yellowish colour.