Preheat the oven to 350 degrees F and line an 8-inch square baking pan with parchment paper.

Add all of the ingredients for the shortbread crust and topping to a large bowl or mixing bowl and mix well until the ingredients are combined. No need to separate the dry ingredients from the wet ingredients - they can all be mixed together simultaneously. The mixture should resemble a fairly dry cookie dough.

Measure out ½ cup of the dough and set it aside to use as the crumble topping.

Transfer the remaining crust mixture into the parchment-lined pan and press it into an even layer. Poke several holes into the dough with a fork.

Bake the crust on the center rack of the preheated oven for 10-12 minutes or until the crust is golden brown around the edges.

Add all of the ingredients for the blackberry filling to a medium saucepan and mix well. Cover the pan and heat over medium-high heat and bring the mixture to a full boil. Once boiling, reduce the heat to medium and mash the blackberries into smaller pieces using a fork or back of a spoon. Continue cooking and stirring until the filling is thick, about 3 to 5 minutes.

Pour the blackberry filling mixture over the prepared crust and spread it evenly over the crust.

Sprinkle the remaining crumble mixture over the blackberry layer.

Bake bars on the center rack (still at 350) for 20 to 25 minutes, or until the crumb topping is golden brown and the blackberry mixture is bubbly.

Remove the bars from the oven and allow them to cool completely before slicing and serving. To speed up this process, you can allow the bars to cool for 10-15 minutes (until the pan is cool enough to touch), then transfer your bars to the refrigerator to chill the rest of the way.

Transfer the bars to a cutting board and use a sharp knife to cut individual bars.

Store bars in an airtight container in the refrigerator for up to 7 days. The bars can be stored at room temperature covered in plastic wrap, but any longer than 2 days, I would recommend refrigeration. You can also freeze the bars in a freezer bag or freezer-safe container for up to 3 months.