Picking berries are one of my favorite chores to do, although I have to admit it was a little more "dangerous" with the possibility of snakes hiding among the brambles! Still, the end product was always worth the danger.
When we lived in the Pacific Northwest, these berries are called Marion berries and I've made lots of jam using them. I've included the recipe for making the jam ahead of time, but you could certainly use "store bought" to shorten the time of preparation. I think these bars are great on their own or with a scoop of vanilla ice cream and they can be frozen, which makes them perfect to do ahead of your picnic.
Blackberry Jam Bars
8 cups fresh blackberries (it was about 3 quarts) rinsed
3 cups (600g) granulated sugar
2 T (30g) fresh lemon juice
In a large saucepan (I used a 6 qt. Dutch Oven), stir together the blackberries, sugar, and lemon juice together. Let stand for 2 hours.
Bring mixture to a boil over medium-high heat. Remove from the heat and use a hand blender to process until smooth. Strain mixture through a fine-mesh sieve, discarding seeds/solids. Pour liquid back into the saucepan.
Cook over medium-high heat for 5 minutes, stirring frequently. Reduce heat to medium and cook for 20-40 minutes until mixture thickens. Remove from heat and allow to cool at room temperature for 1 hour. Transfer to a storage dish and refrigerate for at least 4 hours before using. Makes 2 1/2 cups.
2 cups (4 sticks) (454g) unsalted butter, softened at room temperature
2 cups (400g) granulated sugar
4 large egg yolks
2 tsp. vanilla extract
4 cups (500g) King Arthur all-purpose flour
2 tsp. (10g) baking powder
1/4 tsp. salt
1 1/4 cups blackberry jam
1/4 cup (30g) confectioners' sugar
In a bowl of a stand mixer, using the paddle attachment, cream the butter and sugar together at medium speed until creamy, 3 to 4 minutes. Scrape down the sides of the bowl with a rubber spatula as needed. Beat in vanilla.
Whisk together the flour, baking powder and salt and add to the butter mixture, slowly on low speed until combined. Divide the dough into 4 pieces and wrap in plastic wrap.
Refrigerate for 1 hour.
Preheat oven to 350F-degrees. Line a 13" x 9" baking pan with parchment paper, overlapping the sides and spray the bottom and sides with a baking spray with flour.
Remove two pieces of dough and use a box grater to grate the dough into the pan. Press down slightly on the dough. Place in the freezer for 10 minutes.
Grate the other two disks on a parchment lined baking sheet and freeze for 10 minutes. Remove baking pan from freezer and spread the jam across the top.