Preheat the oven to 350 and grease a 9 x 13 cake pan – for perfect squares, I’d also recommend lining the pan with parchment and buttering the parchment.  In a tiny bowl combine the 2 topping ingredients and set aside.

In a large bowl, use a pastry cutter (or a couple of forks) to cut the butter into the flour(s) and brown sugar – work at it until it’s well combined and the mixture is crumbly.  Press half of this mixture into your prepared pan, I use the back of a measuring cup.

Add to the remaining mixture the baking soda, egg and sour cream and mix well – using a spatula works just fine, and then spread this over the ‘crust’ you’ve already pressed into the pan.  It’s a lot like fluffy muffin batter so you have to be quick.  Get the top as even as you can, it will level nicely as it cooks.

Sprinkle the ‘topping’ evenly over it all and bake for 20 to 25 minutes until it is set, the top should spring back to a light touch.  We think it’s best served a bit warm, but it’s also lovely the next day (if there’s any leftover).