For the jam:
- 2½ cups blackberries, divided
- ¾ cup granulated sugar
- 2 teaspoons lemon zest
- 3 tablespoons lemon juice
- 8 ounces mascarpone
- ½ cup powdered sugar
- Zest of 1 lemon
- ¼ cup lemon juice
- 2 cups heavy whipping cream
- 16–18 graham crackers
- For the jam: Add 2 cups of blackberries, sugar, and lemon zest to a medium saucepan over medium heat. Cook, stirring occasionally, until the blackberries begin to burst and the sugar melts. Use a large spoon or fork to press down on the berries to help release their juices. Turn the heat up to medium-high and cook for another 7-10 minutes until jam thickens and reduces to half the volume. Remove from heat, stir in lemon juice, then allow to cool completely.
- Prepare a 9”x5” loaf pan with plastic wrap, leaving at least a 5 inches of overhang on each side.
- With an electric hand or standing mixer, whip mascarpone, lemon juice, powdered sugar, and lemon zest on medium speed until light and fluffy, about 5 minutes. With the mixer running on low, stream in the cream and beat until medium-stiff peaks form.
- Arrange a single layer of graham crackers along the bottom of the pan, cutting the crackers to fit as needed. Add a ⅕ of the mascarpone mixture on top and smooth out with a spatula. Dollop a ⅓ of the jam on top of the mascarpone and evenly distribute with a clean spatula. Add another ⅕ of the mascarpone over the jam and smooth with the same spatula as the first round. Add another layer of graham crackers over top and repeat layering the mascarpone mixture and jam one more time. Finish by covering the last layer of mascarpone with a layer of graham crackers. Wrap the icebox cake with the plastic overhang and freeze for at least 8 hours or overnight. Transfer remaining jam and mascarpone mixture into airtight containers and refrigerate.
- Remove the jam and mascarpone mixtures from the refrigerator. Slice the remaining ½ cup of blackberries in half and mix them into the jam.
- Remove the icebox cake from the freezer and unwrap the top. Transfer the cake onto a serving plate. Remove the loaf tin and the remaining plastic wrap. With a spatula, evenly “frost” the entire cake with the remaining mascarpone mixture. Transfer the cake back to the freezer for 15-20 minutes. Top cake with remaining jam and slice to serve.
Icebox cake can be fully made and frozen for up to 3 months. Simply thaw for 5-10 minutes before serving.