Preparation Inline ingredients (beta)
Cook mode
Step 1 Preheat the oven to 350F and grease and line a 9×13 baking dish.
Step 2 In a medium bowl, whisk together the flour, cornstarch, baking powder, and salt.
Step 3 In a large mixing bowl using a hand or stand mixer with the paddle attachment, mix together the butter and sugars until pale and fluffy.
Step 4 Mix in the eggs, vanilla, and sour cream until smooth.
Step 5 Pour in half of the dry ingredients and mix to combine. Once smooth, mix in the rest of the dry ingredients to form a thick and smooth dough. It should almost look like creamy mashed potatoes.
Step 6 Spread the dough in the prepared pan and bake for 20-23 minutes or until the edges are a very light golden color and the middle no longer appears wet.
Step 7 Allow the bars to cool completely before frosting. This can either be at room temperature or in the refrigerator.
Step 8 In a large bowl using a hand or stand mixer with the paddle attachment, mix the butter until smooth.
Step 9 Then mix in the powdered sugar one cup at a time, adding in the vanilla and milk when the frosting starts to get dry and crumbly.
Step 10 Once smooth, scoop out 1/3 of the frosting and place in a separate bowl. To this, add a couple drops of green gel food coloring and mix to combine.
Step 11 For the Christmas tree design, cut out a 9×13 sheet of parchment paper. Fold it in half lengthwise and draw out one half of a Christmas tree on one side. With the paper still folded, cut along the outline and then unfold the paper to reveal a perfectly symmetrical tree.
Step 12 Place the tree in the center of the cooled bars and very lightly dust the outline with powdered sugar. Remove the tree to reveal a sugar-dusted outline.
Step 13 Place the green frosting in a piping bag fitted with a small star tip. Dot the buttercream inside the outline to form our tree. Top with sprinkles and brush away any excess sprinkles that are left on the blank surface of the bars.
Step 14 Now place the white frosting in a piping bag fitted with a small star tip. Fill in the remaining surface area, piping as close to the tree outline as possible to create a sharp border.
Step 15 The bars are now ready to slice and enjoy!
