Switch the flat beater to a dough hook and return the bowl to the mixer. Mix on low for a minute then on medium for 5 minutes until the dough looks to be coming away from the sides. Remove the bowl from the mixer scrape the bowl and the dough hook then sprinkle over a spoonful of flour and cover the bowl with plastic wrap. Allow to rise for 1 hour until doubled.
PS. If you double the butter, the topping is so good you could just make that and eat the whole lot out of the saucepan full!