1. Heat oven to 350 degrees F; place rack in center. Line a 13-by-9-inch baking pan with parchment paper, allowing paper to extend on two sides.
2. In a mixer bowl, beat together butter and confectioners' sugar on medium speed until light, about 2 minutes. Beat in extract, then flour and salt until combined well. Press along bottom and 1/2 inch up sides of prepared pan. Bake until just golden brown around edges, 12 to 18 minutes. Transfer pan to a wire rack and let cool completely.
3. Meanwhile, in a medium saucepan, combine butter, brown sugar, milk and salt. Bring to a boil over medium heat, stirring constantly. Reduce heat to low and gently simmer, stirring constantly, until thickened and a deep caramel color, about 15 minutes. Pour over crust and spread evenly; refrigerate until completely cool, about 1 hour.
4. Melt semisweet chocolate; let cool slightly. Using a spatula, evenly spread over filling (for swirled top, see below). Refrigerate until set, about 1 hour. Using a knife, score chocolate layer into 1-12-inch squares.
For swirled top::
5. Divide white chocolate into three parts. Tint one part red and one part green. Leave remaining part white. Dollop red, green and white chocolates over semisweet layer. Using a toothpick, gently swirl together colors.
6. Using white chocolate, adhere candies to squares, making holly or wreaths. Using a small, clean paintbrush, make a snowman out of white chocolate; decorate with candy.
7. Let stand until set, about 20 minutes.
8. Run a sharp knife under hot water; dry. Following score marks, cut into squares. Wipe knife after each cut (run under hot water and dry as needed). (Can be made ahead. Store in an airtight container in the refrigerator up to 1 week.)