Line a large baking tray with baking parchment. Heat oven to 220C/200C fan/gas 7. Heat the butter in a non-stick pan with 110ml water until melted. Increase the heat until boiling, then place on the lowest heat possible and quickly beat in the flour until the mixture comes together as a ball. Take off the heat, beat in the eggs one at a time until glossy and thick, then leave to cool for a few mins.
Spoon or pipe the pastry onto the baking tray in eight equal-sized blobs (about 6-7cm wide), bake for 5 mins, then reduce the oven to 200C/180C fan/gas 6 and continue to bake until well risen and golden. Turn the oven off and leave the buns inside the oven to dry out until cool. Can be made a day ahead and kept in an airtight container.
For the sauce, tip the sugar, syrup, treacle and butter into a pan, bring to the boil and stir until combined. Pour in the double cream and bubble briskly for 2-3 mins until thickened slightly. Stir and season with a pinch of salt. The sauce can be made two days ahead and kept in an airtight container in a cool place. Reheat gently in the microwave or a saucepan before serving.
To serve, cut the buns in half, then place a ball of ice cream on the bottom half. Pop the lid back on and arrange the buns on a big board. Drizzle with the warm toffee sauce and serve any toppings on the side for people to add themselves.