Prepare a mesh strainer by setting it over a medium heatproof bowl. Set the strainer and bowl aside near your stove.
Fill a medium saucepan with 3-4 inches of water and bring it to a simmer over medium heat.
While the water is heating, combine the allulose, lemon juice, and lemon zest in a large, heatproof bowl and whisk to combine. Add the eggs and egg yolks and immediately whisk to combine.
Place the heatproof bowl over the simmering water to form a double boiler. Continue whisking the egg mixture constantly. The allulose will gradually dissolve and then the mixture will begin to thicken. The curd is done cooking when it has reached 160 degrees F and is the consistency of sour cream or full-fat yogurt.
Turn off the burner and remove the bowl from the double boiler, being careful not to burn yourself on the escaping steam. Immediately strain the hot lemon curd through the prepared mesh strainer and into the bowl. Tap the strainer firmly against your hand, and use a rubber spatula to help the curd through without damaging your strainer.
Add the butter, vanilla extract, and salt to the hot curd. Whisk the curd occasionally, until the butter has melted completely and is fully incorporated.
Pour the curd into the parbaked low carb shortbread crust. Gently shake the pan to even out the curd or spread it with an offset spatula. Bake the tart at 325 degrees F for 15 minutes to set the curd filling. For best results, allow the tart to cool and then chill for 1-2 hours, or overnight. Store in the refrigerator.