Strawberry-Rhubarb Oatmeal Cream Pies

Directions

1. Make the cookies: Heat the oven to 350°F with the racks in the upper and lower thirds of the oven. Line two baking sheets with parchment paper or silicone baking mats.
2. In the bowl of an electric mixer fitted with the paddle attachment, cream the butter, brown sugar, and sugar on medium speed until light and fluffy, 4 to 5 minutes. (Note: This can also be done by hand in a medium bowl with a spatula—it just may take a bit longer!) Scrape the bowl and paddle well.
3. Add the eggs one at a time, mixing on medium speed to combine, then scraping well after each is fully incorporated. Add the vanilla and mix to combine.
4. In a medium bowl, whisk the flour, oats, baking soda, salt, cinnamon, and ginger to combine. Add the dry ingredients to the mixer in 2 to 3 additions, mixing each to incorporate, then scraping the bowl well.
5. For large sandwich cookies, roll the dough into 1 inch balls and place on the baking sheet (you should get 24 pieces of dough). Transfer to a baking sheet, staggering the pieces and leaving at least 2 inches around each cookie on all sides, 12 cookies per sheet.
   
   For small sandwich cookies, roll the dough into ½ inch balls and place on the baking sheet (you should get 48 pieces of dough). Transfer to a baking sheet, staggering the pieces and leaving at least 1 inch around each cookie on all sides, 24 cookies per sheet.
6. Bake the cookies until they are lightly golden around the edges, 9 to 12 minutes for large cookies and 6 to 9 minutes for small cookies. Rotate the pans halfway between the oven racks halfway through baking to encourage even browning. Cool completely on the baking sheet.
7. Make the cream filling: in a medium pot, toss the strawberries and rhubarb with the sugar to combine. Cook over medium heat, stirring occasionally, until the fruit becomes very soft and breaks down, 4 to 6 minutes.
8. Strain the mixture through a sieve, reserving the juices in a liquid measuring cup and discarding the fibrous solids.
9. In the bowl of an electric mixer fitted with the paddle attachment, cream the butter, cream cheese, vanilla paste (or extract), and salt until light, about 2 minutes. Add the powdered sugar in 2 to 3 additions, mixing each until combined and scraping the bowl well.
10. Add the reserved strawberry-rhubarb juice and the mascarpone and mix until the mixture is smooth. If desired, tint the filling with a drop or two of red food coloring.
11. Flip over half of the cookies, and use a small offset spatula to spread the filling on top of each of the flipped cookies. (If desired, you can also transfer the filling to a pastry bag with ½ inch opening cut from the tip, or fitted with a star or round pastry tip to pipe the filling inside for an attractive effect.)
12. Press the remaining cookies on top, forming the sandwiches. Store cookies in an airtight container at room temperature for up to 24 hours, or refrigerate for up to 5 days.