Step 1: Prepare Cookie Dough. In a medium bowl, whisk together the flour, baking powder, baking soda, cornstarch, and salt until evenly distributed. In a separate, large bowl, use an electric mixer to cream together the softened butter and granulated sugar until light and fluffy (2-3 minutes). Beat in the egg, sour cream, vanilla extract, and almond extract until fully combined and smooth. Gradually add the dry ingredients to the wet mixture, mixing on low speed until just combined. Do not overmix.
Step 2: Chill Dough. Divide the dough in half, flatten each half into a disc, and wrap snugly in plastic wrap. Refrigerate for at least 1 hour, or up to 24 hours.
Step 3: Bake Cookies. Preheat oven to 350°F (175°C). Line baking sheets with parchment paper. On a lightly floured surface, roll one disc of chilled dough to approximately ¼-inch thickness. Use Halloween-themed cookie cutters to cut out shapes. Carefully transfer cut-out cookies to the prepared baking sheets, leaving about an inch of space between each. Bake for 8-10 minutes, or until the edges are a light golden brown and the centers remain pale. Let cookies cool on the baking sheets for a few minutes before moving to a wire rack to cool completely.
Step 4: Prepare Frosting. While cookies cool, make the buttercream. In a large bowl, beat the softened butter until creamy and smooth. Gradually add the powdered sugar, alternating with the milk, beating until the frosting is smooth and fluffy. Stir in the vanilla extract. Divide the frosting into separate bowls (e.g., three) and add a few drops of orange, purple, and green food coloring to each bowl, mixing until desired vibrant hues are reached.
Step 5: Decorate. Ensure cookies are completely cool before frosting. Generously spread or pipe the colored frosting onto each cookie. Garnish with Halloween sprinkles before the frosting sets. Serve immediately and enjoy!