Preheat the oven to 375 degrees Fahrenheit and generously grease an 8-inch tart pan

In the bowl of a stand mixer, cream the butter and sugar on medium speed for about 5 minutes, until pale and smooth

Slowly add the flour and salt until it becomes a crumbly dough

Press the dough into the bottom of the prepared tart pan in an even layer

Bake for about 20 minutes, until the tart crust is lightly golden brown

Allow the crust to cool to room temperature

Add 1 inch of water to a small saucepan that will fit a metal bowl or double boiler and bring to a simmer over medium heat

In a small bowl, whisk the egg yolks and sugar until smooth

Whisk in the lemon juice and lemon zest until combined

Place the lemon curd mixture in a double boiler and stir continuously for about 10 minutes, until the mixture thickens and coats the back of a spoon, making sure to scrape the sides of the pan as you stir

Turn off the heat then whisk in the butter, a little bit at a time

Spread the blackberry preserves in an even layer on top of the shortbread crust tart shell

Pour the warm lemon curd tart filling on top of the blackberry layer and smooth into an even layer

Chill the tart in the refrigerator for at least one hour

Garnish with more blackberry preserve and powdered sugar and serve cool