To make homemade jam, follow our recipe here. It's a good idea to make this in advance so it has time to cool before using it in the hindbærsnitter. Combine flour, powdered sugar, and salt in a food processor and pulse to combine. If you don't have a food processor, you can also do this by hand in a bowl.
Add cold butter and pulse in the food processor until it looks like sand. If doing this by hand, just break up the cold butter with your fingers and crumble until you reach a sand-like consistency.
Add egg and vanilla extract and pulse until the dough starts to come together. If working by hand, add to your bowl, mix to distribute, and gently press together until your dough forms a ball.
Wrap the dough in plastic wrap and refrigerate for at least 30 minutes.
Preheat your oven to 375 degrees F (190 C). Line a baking sheet with parchment paper.
After refrigerating, roll out about two thirds of your dough into a rectangle to form the base of the pastry, about 1/4 inch thick.
Slide this base onto the baking sheet or roll the dough onto your rolling pin, then unroll onto the baking sheet for easy transfer!
Spread the jam in a thin layer on top of the dough, all the way to the edges.
Roll out the last third of the dough into a rectangle, and cut into strips of equal width (between 3/4" and 1"). If you want to be precise about the length, you will need 4 long strips (the same length as the long side of the pastry base) and 6 short strips (the same length as the short side of the pastry base). Honestly, though, we just cut a bunch of short strips and then perform some pastry surgery to make the long strips. It's way easier and will still look great when it's baked!
Make your lattice - this looks way harder than it actually is! Start by laying down all of the short strips on top of the jam layer.
Now, fold back the second, fourth, and sixth short strips, so that they're all still attached along the left side of the pastry but there's room to lay down the first long strip. Lay down the long strip (farthest to the left in the picture) so that it goes over, under, over, under, over, under, etc all the short strips.
Then, fold the second, fourth, and sixth strips back to where they were. Fold back the first, third, and fifth strips. Lay down the next long strip - this time the pattern should be under, over, under, over, etc.
Fold the first, third, and fifth strips back to where they were. Repeat this process with the rest of the long strips, alternating which short strips you fold back.
Once you are done, the pattern should alternate and this creates the lattice look!
At the end, we like to take the remaining dough scraps and roll out very thin strips to cover the edge. This is definitely optional, it just makes the edges neater!
Bake for 10-15 minutes, until lightly golden brown on top and around the edges.
Let cool briefly before slicing. We divide the rectangle into 12 square (ish) slices. Enjoy!