To Make Strawberry-Rhubarb Filling:

In a large saucepan over medium heat, combine all filling ingredients, except for cornstarch and water. Combine 1 tsp. cornstarch with 1 tsp. cold water and stir to dissolve.  Add to strawberry rhubarb mixture.  Bring to a simmering boil.  Cook until fruit is soft and reduce to about half.  Combine remaining 1 tsp. cornstarch with 1 tsp, cold water and add to simmering fruit mixture.  Cook until mixture coats the back of a spoon and leaves a distinct line when run over with another spoon or finger.  Set aside to cool. 

To Make Lemon Cookies:

Preheat oven to 350 degrees F.  Line 2 baking sheets with parchment paper.  Set aside. 

Using a standing or hand-held mixer, cream butter and sugars together creamy and fluffy.  Add zest, egg, vanilla, and lemon extract/oil.  Beat until evenly combined. 

Whisk together flour, salt, and baking powder.  Gradually add to egg mixture until evenly combine and flour is just incorporated.  Use a #16 cookie scoop or 1/4-cup measuring cup to portion dough out onto the prepared baking sheets.  Space dough about 2-inches apart.  Press dough down into about a 3/4-inch thickness.  Bake for 12-14 minutes or until edges of cookies are just beginning to brown.  Remove sheets from oven and set onto cooling racks.  Use a 1/4-cup measuring cup to press an indent into each cookie while still warm.  Allow to cool completely.  

Once cooled, place about 1 heaping T. of Strawberry-Rhubarb Filling into each indentation.  If desired, drizzle melted white chocolate over each cookie, or dust with powdered sugar.