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1
Preheat the oven to 350F.
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2
To make the crust, mix together the baking mix, xanthan gum, and salt in a medium bowl.
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3
In the bowl of a stand mixer, cream together the shortening and sugars, Stir in the vanilla, Mix in the dry ingredients until they are thoroughly combined, then press the dough into a 9x 13-inch baking pan.
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4
Chill the crust in the refrigerator for 20 minutes.
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5
Remove the crust from the refrigerator and prick it all over with a fork.
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6
Bake the crust for 20 minutes, or until lightly browned.
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7
Transfer the baked crust from the oven to cooling racks and cool for at least 30 minutes.
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8
Maintain the oven temperature.
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9
While the crust is cooling, make the filling.
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10
In a medium saucepan, whisk together 3 cups of the confectioners sugar and the cornstarch.
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11
Continue whisking and slowly add the water.
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12
Whisking constantly, bring the mixture to a full boil over medium-high heat (8 to 10 minutes), Sprinkle the gelatin over the boiling mixture and stir to dissolve it.
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13
Reduce the heat to medium and continue cooking, stirring constantly, for another 5 minutes; the mixture should be quite thick, Remove the custard from the heat and stir in the lemon juice and zest.
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14
Pour the lemon custard mixture over the cooled crust.
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15
Bake for 30 minutes.
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16
Remove from the oven and let cool completely in the pan on a cooling rack, Dust with confectioners sugar and cut into squares for serving.
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17
Store the lemon squares in an airtight container and refrigerated for up to 3 days.
