Make your kagecreme in advance so it can fully chill in the fridge before use. Combine flour, powdered sugar, and salt in a food processor and pulse to combine. You can also do this by hand in a separate 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 it reaches 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). After refrigerating, roll out dough until it is about 1/4 inch thick.
Cut out circles about 2 inches in diameter. Repeat until you have used up all the dough.
Transfer your cookies to a baking sheet lined with parchment paper. Poke the tops with a fork to prevent uneven rising.
Bake for 10-15 minutes until lightly golden brown around the edges. Let your cookies cool completely.
Once your cookies are cooled, use a regular teaspoon to spread kagecreme (pastry cream) onto one side of a cookie and add a second cookie to sandwich it together. They are great eaten immediately, but if you let the cookies set in the fridge overnight, they will hold their shape even better!
Optional, but recommended: Make a thick icing by mixing powdered sugar with a tiny bit of water. Spread a small amount of icing onto each cookie and top with a fresh raspberry. Enjoy!