Bring the butter, jam, and sushi vinegar to room temperature. Combine the cake flour and baking powder and sift.
Cream the butter until smooth, add the granulated sugar and mix until light and fluffy. Then add the jam and mix well.
Add the cake flour and baking powder and fold it in. While it still looks floury, add the sushi vinegar and mix.
Wrap in plastic wrap and roll into a 3 - 3.5 cm cylinder. Let the dough rest in the refrigerator for about 30 minutes.
Preheat the oven to 170ºC. Cut the dough into 5-6 mm thick slices and place on a baking pan lined with parchment paper.
Bake at 170ºC for 14-15 minutes. When they are done baking, cool the cookies on a cooling rack.
These cookies won't be soft; they will have a crunchy texture. While they're baking, the sweet scent of the strawberry jam will fill the room.
Here they are in a soft, moist galette style. Increase the amount of butter to 50 g, add 50 ml of milk at step (3) and pour into a muffin pan. Bake at 180ºC for 30 minutes, then decrease the oven to 150ºC and bake for 10 minutes.
Story Behind this Recipe
The sushi vinegar I used isn't just sour; it has a nice balance of sweet and umami, so I thought it would work well for making sweets. This was one of the recipes I came up with. Personally, I like the soft, moist galette-style version described in Step 8. But the hard to please judges at home (my kids) prefer the crunchy cookie version, so I posted this one.
Depending on the jam you use, the dough might be too soft even after you let it rest in the refrigerator. If that happens it will be hard to cut the dough in Step 5, so you can pipe the dough or use a spoon to drop it onto the baking pan.