Preheat oven to 350°. Line an 8-inch square baking pan with foil, letting excess extend over sides of pan. Spray foil with cooking spray.
In the work bowl of a food processor, pulse together 1⁄4 cup granulated sugar and lavender until lavender is finely chopped.
In a large bowl, beat together butter, remaining 1 cup granulated sugar, and lavender sugar with a mixer at medium speed until fluffy, 3 to 4 minutes, stopping to scrape down sides of bowl. Beat in egg, egg yolk, and vanilla extract, one at a time, until combined.
In a medium bowl, whisk together flour, baking powder, and salt. With mixer at low speed, gradually add flour mixture to butter mixture, beating until combined. Stir in white chocolate just until incorporated. Spread batter into prepared pan.
Bake until a wooden pick inserted in center comes out clean, 20 to 25 minutes. Let cool completely in pan.
Using excess foil as handles, remove blondie from pan and place on a cutting surface. Remove and discard foil. Using a large sharp knife, trim edges from blondie to form a 7-inch square. Cut into 16 (1 3⁄4-inch) squares. Serve immediately, or store at room temperature in an airtight container and serve within 3 days.
If desired, just before serving, place an X-shaped stencil* on top of each blondie square with points touching corners of squares. Dust with confectioners’ sugar. Carefully lift stencil to remove.
Notes
*Trace X pattern onto sturdy paper, and cut out.
Recipe by TeaTime Magazine at https://teatimemagazine.com/lavender-white-chocolate-blondies/