Preheat oven to 350°F. Line a second cookie sheet with parchment paper or a silicone baking mat.
Place softened butter in the bowl of a stand mixer fitted with the paddle attachment (or a large bowl if using a hand mixer). Add granulated and brown sugars and mix on low speed until the mixture is fluffy. Mix in egg and cake batter extract and mix on medium speed until combined.
Mix in baking powder and salt, then slowly mix in flour and mix just until the batter is smooth and comes together. Be sure to scrape the sides of the bowl during mixing. Slowly mix in sprinkles.
Line a cookie sheet with a silicone baking mat or parchment paper. Scoop 2 or 3 tablespoon balls of dough onto the cookie sheet. Space them about 3 or 4 inches apart, depending on size. Press them gently with the bottom of a drinking glass to flatten.
Bake 13-16 minutes for 2-tablespoon size or 18-20 for 3 tablespoon size. The edges will be a light golden and the tops will no longer be glossy. Let cool completely before frosting.
To make frosting: Beat butter until smooth with a stand or hand mixer in a very large bowl. Slowly mix in powdered sugar until crumbly, then add salt and extract. Add 2 tablespoons heavy cream and mix, until smooth and fluffy, adding more cream as needed for desired consistency.
Frost the cookies as desired and top with more sprinkles. To frost them like Crumbl does, use a pastry bag with a round tip and spiral the frosting.
Store cookies in an airtight container for up to 3 days. Freeze for up to 3 months.