Start by making the blackberry jam layer. Set a skillet or wide sauce pan over medium heat. Add the blackberries, lime juice, and sugar and stir to coat the berries in the sugar. Once the berries start to release their juices and soften, begin to gently mash them to encourage them to break down. Once the berries are mostly broken down, stir the mixture every minute or two to prevent it from burning. The jam is ready once the juices have reduced and thickened enough that you can scrape your spoon across the bottom of the pan and the gap it leaves doesn’t immediately fill back in with jam. This will take about  15-20 minutes total. Once the jam is ready, remove it from the heat and stir in the lime zest.

Pre-heat your oven to 350F degrees. Line an 8×8-inch square baking pan with parchment paper, leaving an overhang. In a large mixing bowl, whisk together the flour, salt, and dark brown sugar. Using your fingers, break up any clumps of brown sugar. Stir in the melted butter and vanilla until you have a uniform dough. Add roughly half of the dough to your parchment lined pan and pat it into an even layer. If it doesn’t look like enough, add a bit more dough. You want an even layer that covers the entire bottom of the pan and is roughly 1/4-inch thick.

Next, spoon all of the jam onto the dough. Gently spread it into an even layer. Then, crumble the remaining dough over the top of the jam with your fingers, making sure to evenly distribute it. Bake the bars in your pre-heated 350F-degree oven for 35-40 minutes or until the juices of the jam are bubbling and the crumble on top is baked through and golden.

Allow the bars to cool for 30 minutes before slicing. For larger bars, cut into 9 squares. For smaller bars, cut into 16 squares. Before serving, you can sprinkle some lime zest over the top of the bars if you like.

Store leftovers in an airtight container in the fridge. Best if enjoyed within 2-3 days.