Preheat the oven to 350°F and line an 8-inch square pan.
Mix the almond flour, coconut flour, sweetener, and salt.
Cut in the butter until crumbly, then stir in the egg and vanilla to form a dough.
Press two-thirds of the dough into the pan and bake for 12 minutes, until pale golden.
Mash the raspberries and stir in the lemon juice, zest, and chia seeds. Rest for 10 minutes to thicken.
Spread the fruit mixture over the baked base and crumble the remaining dough on top.
Bake for 15 to 18 minutes, until set. Cool completely, refrigerate until firm, and slice.