Preheat the oven to 180ºC. Lightly grease and line a 9 x 13 - inch (22 x 33 cm) tin with parchment paper.
Mix together then cinnamon, sugars and Colman’s Mustard Powder
In a large bowl, stir together the flour, bicarbonate of soda, baking powder and salt. In a small bowl, stir together the sour cream, honey and mustard.
Beat the butter and sugar together with an electric hand whisk until light and fluffy.
Add the eggs, one at a time, beating well after each addition.
Add one third of the flour mixture, and mix on medium-low speed until just combined. Add half of the sour cream mixture. Repeat with the remaining flour and sour cream, stopping to scrape the bowl now and again.
Transfer half of the batter to the prepared tin and spread it out evenly. Sprinkle with half of the cinnamon mustard sugar. Repeat with the remaining batter and cinnamon sugar.
Bake for 30 -35 minutes until a skewer inserted into the middle of the cake comes out clean. Remove from the oven and let cool.
Stir together the icing sugar and milk. Drizzle over the cooled coffee cake. Slice and serve.