In a large mixing bowl, whisk together the all-purpose flour, baking powder, and salt until well combined. In another bowl, cream the first portion of unsalted butter with granulated sugar using a hand mixer on medium speed for about 3-4 minutes, until light and fluffy. Add in the egg, vanilla extract, and lemon zest, mixing thoroughly. Gradually incorporate the dry flour mixture, blending gently until just combined to ensure a tender texture in your Lemon Lavender Sandwich Cookies.

Divide the cookie dough into two equal portions and wrap each in plastic wrap. Refrigerate the dough for at least 30 minutes to firm up, which makes rolling easier. During this time, the flavors will meld beautifully, creating an exquisite base for your sandwiches.

Preheat your oven to 350°F (175°C) while you roll out the chilled dough on a lightly floured surface. Use a rolling pin to roll to about 1/4-inch thickness. Cut the dough into your desired shapes with cookie cutters, then place the cut-outs onto lined baking sheets.

Bake the cookies in the preheated oven for about 10-12 minutes, or until the edges are lightly golden. Remove them from the oven and allow the cookies to cool completely on the baking sheets for about 5 minutes before transferring them to wire racks.

While the cookies cool, prepare the frosting by beating the second portion of unsalted butter in a medium bowl with powdered sugar, finely ground culinary lavender, heavy cream, and vanilla extract. Mix on low speed until combined, then increase to medium-high for 2-3 minutes until fluffy.

Once the cookies are completely cool, spread an even layer of the lavender frosting on the flat side of one cookie and gently press another cookie on top to create a sandwich. Repeat until all cookies are assembled, garnishing with thin lemon slices and sprigs of fresh lavender.