• Make the blackberry compote first, followed by the shortbread crust and then the lemon filling. While your tart is baking, you can make the optional candied lemon for a decorative topping.

Blackberry compote

• Combine the blackberries, sugar, and lemon juice in a medium saucepan.
• Cook over medium-high heat for 20-25 minutes. Stir as the berries start to break down and thicken.
• Cook the berries until they have reduced and thickened to a jam-like consistency. Set the sauce aside to cool. It will continue to thicken slightly as it cools down.

Shortbread crust

• Preheat the oven to 350ºF.
• Add the butter, sugar, and salt to the bowl of a stand mixer. With a paddle attachment, cream everything together on medium speed until it is light and fluffy, 2-3 minutes. Scrape down the sides of the bowl.
• Add the egg yolk and vanilla extract. Mix until just combined. Scrape down the sides of the bowl.
• With the mixer on low speed, slowly add the flour, and mix until everything is just combined. Be careful not to overmix.
• Form the dough into a rough ball. Place it on a lightly floured surface. I prefer a piece of parchment paper to make transferring your rolled-out dough to the tart pan easier. Roll the dough into a disk that is approximately a quarter of an inch thick.
• Place your dough into a 10-inch tart pan. Work the dough into all the corners and sides with your hands. The bottom of a measuring cup can also be used to help get it into all the nooks and crannies. If there is extra dough sticking up above the tart pan, trim it with a knife so that it is level with the top of the pan. Use a fork to poke holes in the bottom of the tart dough.
• Place the pan into the oven and bake for 10 minutes at 350º.

Lemon filling

• While the tart dough is baking, make the lemon filling.
• Combine the sweetened condensed milk, lemon juice, lemon zest, and egg yolk in a medium-sized bowl. With a hand mixer, combine the ingredients until everything is well incorporated. Set aside.

Assemble the tart

• Pour the lemon filling into the warm tart shell and spread it edge to edge.
• Swirl the blackberry sauce through the lemon filling. If your sauce has cooled and become too thick, the filling can be microwaved for 20-25 seconds to thin it out.
• Place the tart in the oven and bake for 10-15 minutes at 350º or until the crust becomes golden brown. The filling should not jiggle in the center when the pan is bumped. Be careful not to overbake. The filling will continue to firm up in the refrigerator.
• Allow the tart to chill in the refrigerator for at least two hours before serving; overnight is even better. Add the optional garnishes after removing the tart from the fridge before serving.

Optional garnish: candied lemon slices

• Add the water, sugar, and lemon juice to a medium-sized saucepan. Cook over medium heat until the sugar has dissolved.
• Arrange the lemon slices in a single layer in the pan. Simmer everything over medium-low heat for about 15 minutes. Flip the lemon slices about halfway through the cooking process.
• When done, remove the lemon slices and allow them to cool on parchment paper or a wire rack.