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Confetti Sugar Cookie Sandwiches with Vanilla Buttercream

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Ingredients
  

  • All-purpose flour: The base structure for soft, chewy cookies.
  • Cornstarch: Adds tenderness and helps create that soft-bake texture.
  • Baking powder and baking soda: A mix for gentle lift and even spread.
  • Fine salt: Balances sweetness and enhances vanilla flavor.
  • Unsalted butter (room temperature): For rich flavor in both cookies and buttercream.
  • Granulated sugar: Sweetens and helps the cookies bake up with a light, soft crumb.
  • Egg: Binds the dough and adds moisture.
  • Vanilla extract: Use pure vanilla for a warm, classic flavor.
  • Sour cream or plain Greek yogurt (optional): Just a spoonful adds softness and keeps cookies moist.
  • Rainbow jimmies (sprinkles): For that confetti look; avoid nonpareils to prevent color bleed.
  • Confectioners’ sugar (powdered sugar): For a smooth, fluffy buttercream.
  • Heavy cream or milk: To thin the buttercream to a spreadable consistency.
  • Pinch of salt for frosting: Balances sweetness and rounds out the flavor.

Instructions
 

  • Prep the basics: Line two baking sheets with parchment paper. Preheat your oven to 350°F (175°C). Set out the butter so it’s truly room temperature—soft but not greasy.
  • Whisk dry ingredients: In a medium bowl, whisk 2 1/4 cups all-purpose flour, 1 tablespoon cornstarch, 1/2 teaspoon baking powder, 1/4 teaspoon baking soda, and 1/2 teaspoon salt. Set aside.
  • Cream the butter and sugar: In a large bowl, beat 3/4 cup unsalted butter with 3/4 cup granulated sugar on medium speed for 2–3 minutes until light and fluffy. Scrape the bowl as needed.
  • Add egg and vanilla: Beat in 1 large egg and 2 teaspoons vanilla extract until smooth. If using, mix in 2 tablespoons sour cream or yogurt.
  • Add dry ingredients: On low speed, add the flour mixture in two additions. Mix just until combined. Don’t overmix.
  • Fold in sprinkles: Gently stir in 1/2 cup rainbow jimmies. Keep stirring to a minimum to prevent streaking.
  • Chill briefly: Cover and chill the dough for 20–30 minutes. This helps the cookies hold their shape and prevents overspreading.
  • Portion and flatten: Scoop 1 tablespoon portions (about 20–22 cookies total). Roll into balls and place 2 inches apart on the prepared sheets. Lightly press the tops to flatten to about 1/2 inch thick for even baking.
  • Bake: Bake one sheet at a time for 8–10 minutes, until edges are set and centers look slightly underdone. They’ll finish setting as they cool.
  • Cool completely: Let cookies rest on the sheet for 5 minutes, then transfer to a rack to cool. Cool fully before filling.
  • Make the buttercream: Beat 3/4 cup unsalted butter (room temp) until creamy. Add 2 1/2 to 3 cups confectioners’ sugar, 2 teaspoons vanilla extract, a pinch of salt, and 1–3 tablespoons heavy cream or milk. Beat on medium-high for 2–3 minutes until fluffy and smooth. Adjust with more sugar or cream to reach a thick, spreadable consistency.
  • Assemble: Pair cookies by size. Pipe or spread 1–2 tablespoons of buttercream onto the flat side of one cookie. Top with its partner and gently press to spread the filling to the edges.
  • Optional garnish: Roll the edges in extra rainbow jimmies for a party-ready finish.
  • Set the sandwiches: If you want cleaner edges, chill the sandwiches for 15 minutes to help the buttercream firm up before serving.