Pre-heat the oven to 220°C. Sift the flour, salt and a little pepper together in a large mixing bowl. Combine the butter and milk in a medium saucepan and heat gently until the butter has melted. Add the flour to the melted butter and milk mixture and stir until a smooth dough comes together. Continue to beat the dough until it starts to pull away from the sides of the saucepan. Remove from the heat and add the eggs, one at a time, beating well between each addition, until the dough is shiny and even. Carefully transfer the dough to a piping bag fitted with a 2-3cm straight-sided nozzle. Pipe onto a lined baking tray, spaced apart, and bake in the oven for 10 minutes. Reduce the heat after 10 minutes to 150°C and continue to bake for 5-7 minutes until evenly browned and risen. The choux buns are ready when the inside is cooked through and not wet or doughy and the outsides are golden in colour. Remove and allow to cool on a wire rack. Meanwhile, melt the butter in a large saucepan set over a medium heat. Whisk in the flour to create a roux and cook for 1-2 minutes until smooth. Pour in the milk in a slow, steady stream, whisking simultaneously until it is all incorporated. Fold in the diced bacon and mushroom. Simmer the sauce until thickened, stirring occasionally; usually 6-8 minutes. Season to taste, then set to one side. Carefully remove the tops from the choux buns using a sharp knife. Hollow out the centres if need be and spoon the filling into them. Replace the tops and arrange on wire racks lined with nonstick baking paper before serving.