Preheat the oven to 350℉ and line the baking dish for easy removal later, be sure to leave overhang so you can grab and lift the bars from the dish.
Now, chop the peaches up, then mix the sugar, cornstarch and cinnamon with them. Set to the side.
In a bowl, combine the flour, sugar, baking powder, salt and cold butter. A pastry cutter is great here but if not use a mixer until everything is combined. Once combined, take about 90% of the dough and press down into the bottom of the baking dish forming the bottom crust.
Next, pour the peach mix that you set aside on top and spread out evenly. Take the remaining dough and drop crumbles across the top of the peaches. Place the baking dish in the oven and bake for 40 minutes.
Remove from the oven when the time is up and allow the peach bars to cool fully. Next, place the cooled bars into the refrigerator to set for a few hours for easy cutting and serving.
When the time is up, remove the bars from the pan using the lining you placed in early. Cut the bars and then serve.