Preheat oven to 375. Line a 9×13 inch baking pan with foil and lightly grease the foil. In a medium bowl whisk together flour, baking powder, and salt.
In a large bowl cream butter until light and fluffy (about 1 minute). Add sugars and mix until combined. Add egg, sour cream, and vanilla and mix well. Gradually mix flour mixture into wet ingredients and continue to mix until the dough comes together and not crumbly.
Gently press dough into bottom of prepared pan. Bake 18-22 minutes until golden and the top no longer appears “wet”. Allow to cool completely.
When the bars have cooled, prepare the frosting. In a medium bowl whip butter for 2-3 minutes (no less!) until very fluffy. Add cream cheese and mix well. Gradually mix in powdered sugar until completely combined. Mix in peppermint extract. Spread frosting over cooled bars. Sprinkle with crushed candy canes. Cut into squares or triangles and serve. Store in airtight container.