Step 1: Preheat oven to 350 degrees. Whisk together flour, baking soda, and salt; set aside.
Step 3: mix butter and 1 1/2 cups sugar with an electric mixer on medium-high speed until pale and fluffy, 2 to 3 minutes. Add egg, extract, and vanilla seeds; mix until smooth. Reduce speed to low. Gradually mix in flour mixture.
Step 5: scoop batter using a flat (not mounded) 1-inch ice cream scoop; space 2 inches apart on baking sheets lined with parchment paper.
Step 7: bake cookies 4 minutes. Remove from oven; gently tap baking sheets on counter to flatten cookies. Return to oven, switching positions of sheets; bake until cookies are just set, 4 to 6 minutes more. Let cool on parchment on wire racks.
Step 9: puree raspberries and remaining 2 teaspoons sugar in a food processor. Pour mixture through a fine sieve into a small bowl, pressing to extract juice; discard seeds. Set mixture aside.
Step 11: melt white chocolate in a heatproof bowl set over a pan of simmering water. Remove from heat; whisk in cream in a slow stream. Slowly whisk in reserved raspberry mixture. Refrigerate up to 30 minutes.
Step 13: spread 1 heaping teaspoon raspberry cream on the undersides of half the cookies. Sandwich with remaining cookies.