Add the blackberries, sugar, and lemon juice to a small saucepan. Bring to a gentle simmer over medium heat and cook for about 15 minutes, stirring occasionally and pressing the berries against the side of the pan to release their juices. Continue cooking until the mixture has thickened to a jam-like consistency. Remove from the heat and cool completely. For a smoother blackberry swirl, strain out the seeds before cooling if desired.
2 cups (290 g) fresh blackberries, ½ cup (100 g) granulated sugar, 2 tbsps (30 ml) fresh lemon juice
Combine the crushed shortbread cookies, melted butter, and lemon zest in a small bowl. Stir until the crumbs are evenly coated and set aside.
1 cup (100 g) shortbread cookies, 2 tbsps (28 g) unsalted butter, 1 tsp (2 g) lemon zest
In a large mixing bowl, whisk together the sweetened condensed milk, lemon zest, lemon juice, almond extract, and salt until smooth.
1 (14 oz / 396 g) can sweetened condensed milk, 1 tbsp (6 g) lemon zest, 2 tbsps (30 ml) fresh lemon juice, ½ tsp (2.5 ml) almond extract, ¼ tsp (1.5 g) fine sea salt
In a separate chilled bowl, whip the cold heavy cream to stiff peaks. Gently fold the whipped cream into the condensed milk mixture in several additions, using a spatula and a light hand to keep the mixture airy.
2 cups (480 ml) heavy whipping cream
Line a 9 x 5-inch (23 x 13 cm) loaf pan with parchment paper if desired for easier removal, or use any freezer-safe container that allows for layering. Spoon in a layer of the ice cream base, followed by some of the blackberry mixture and a handful of the lemon crumble. Repeat the layers until all of the ingredients have been used, adding the reserved whole blackberries between the layers if using.
Swirl a butter knife through the layers a few times to create blackberry ribbons throughout the ice cream. Avoid overmixing so the swirls remain distinct.
Cover tightly and freeze for at least 8 hours, or until firm enough to scoop.
Let the ice cream sit at room temperature for 5 to 10 minutes before serving. Garnish with fresh blackberries, additional lemon zest, or extra crumble, if desired.