In a medium-size bowl, whisk the soft butter until smooth and creamy. Add the sugar, vanilla and salt and whisk again until well combined. Add the all-purpose and almond flours and stir with a fork until the flour is incorporated and large crumbs start to form.

Spray a 9-inch tart pan (or a 9-inch cake pan) with baking spray. (if using a cake pan, line the bottom with parchment paper for easy removal.) Press approximately one-half of the dough evenly up the sides and covering the bottom of the pan. Place the bowl with the remaining dough into the refrigerator.

Position a rack in the middle of the oven. Place a sheet pan on the rack and preheat the oven to 325 degrees.

Bake the bottom crust on the sheet pan for 10-14 minutes, until just beginning to firm and turn pale golden brown at the edges.

Remove the sheet pan from the oven and allow the tart to cool for 10 minutes, then spread the lemon curd evenly over the bottom of the crust.

Remove the remaining dough from the refrigerator. Grab a handful of the crumb mixture and gently squeeze it in your hand, then sprinkle it over the lemon curd, breaking it up into large crumbs as you go. Repeat until all of the crumb mixture is used up.

Return the tart (on the sheet pan) to the oven and continue baking until topping is firm, crisp and lightly golden in color, about 25-30 minutes. Transfer to a wire rack and let cool to room temperature. Remove the outer ring from the tart pan and serve. Dust with powdered sugar before serving, if desired.