Lightly grease the bottom of a 9x9 baking pan, then put parchment inside. Preheat the oven to 350°F. In a large bowl, combine almond flour, coconut flour, sweetener, lemon zest, xanthan gum and kosher salt and cold butter. Work the butter into the dry ingredients by hand with a dough cutter, or use a food processor if you prefer. The mixture should look like wet sand. You can use your hands at this point to shape the dough into a ball.
Cut the ball of dough in half. Set half aside to press into the bottom of the baking pan. Add the pecans to the remaining half, mixing it together with your finger tips. This will be your crumb topping. Set aside.
Press half of the dough into the bottom of your pan. We use a measuring cup to ensure it’s as even as possible.Bake the crust for about 15-17 minutes, until it’s golden brown. Set aside and allow to cool for about 15-20 minutes before moving on.
In a sauce pan, add plums, lemon juice, remaining butter, lemon zest, sweetener, cinnamon, and salt. Bring to a boil, then reduce to a simmer to allow the plums to soften, break down and thicken for about 15 minutes. Adjust sweetness and flavors while tasting as you go.
After the mixture has began to thicken now that some water has evaporated off, add in the vanilla extract and some xanthan gum in small bits at a time, whisking continuously to avoid clumping. Allow the mixture to simmer for about 20 more minutes.
Add the jammy plum filling to the cooked crust, spreading it evenly. Add your crumble topping, and lightly press it in to the filling to adhere. Bake for an additional 22-27 minutes, until the top is golden brown. Allow the bars to cool completely before cutting. This takes us about 2 hours or more.
Cut into 12 equal pieces. Serve plain, or with some low carb ice cream or whipped cream if you want. Store in an airtight container in the fridge for up to 1 week.