Preheat the oven to 230C/gas mark 8. Butter and flour a 25cm cake tin.
To make the filling cream the butter and cinnamon together in a large bowl or in an electric food mixer until soft. Add the brown sugar and beat until the mixture is light and fluffy and then stir in the chopped toasted pecans. Set aside (not in the fridge).
To make the dough sift the flour, baking powder and cinnamon together into a large bowl. Add the caster sugar, then add the butter, and using your fingertips, rub it in until the mixture resembles breadcrumbs. Whisk the beaten egg and milk together in a separate bowl, then make a well in the centre of the dry ingredients and pour in most of the liquid (all but 60ml). Using one hand with your fingers outstretched like a claw, move your hand around in one direction (clockwise or anti-clockwise) until the dough comes together. You may need to add the rest of the milk as the dough should be soft and a little sticky.
To make the buns, tip the dough out onto a floured work surface and dust with flour. Using a rolling pin, roll out the dough until it is a 35 x 25cm rectangle and about 2cm thick.
Spread the filling all over the rectangle, then with the widest end facing you, roll up the dough away from you so that it resembles a Swiss roll. Cut the ‘log’ 11 times to make 12 slices, each 3cm thick. Place the pieces cut side facing up, with a tiny bit of space between each ‘swirl’, in the prepared cake tin.
Bake in the oven for 10 minutes, then reduce the oven temperature to 200C/gas mark 6 and cook for a further 25–35 minutes or until risen, golden brown and cooked in the centre. They should have joined together to make a lovely cluster of buns.
Allow to stand in the tin for 2–3 minutes before carefully turning out and cooling on a wire rack. When cool, transfer to a serving plate or cake stand.
To make the icing, sift the icing sugar into a bowl, add 1 tablespoon of boiling water and mix well, adding another 1/2–1 tablespoon of boiling water if necessary, until the icing is soft but not too runny. Drizzle the icing over the buns. To serve, break each bun off with your hand or cut into slices.