Heat oven to 350°F; place rack in center. Line a 13-by-9-inch baking pan with parchment paper, allowing paper to extend on two sides.
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 ½ inch up sides of prepared pan. Bake until just golden brown around edges, 12 to 18 min. Transfer pan to a wire rack and let cool completely.
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 min. Pour over crust and spread evenly; refrigerate until completely cool, about 1 hour
Melt semisweet chocolate; let cool slightly. Using a spatula, evenly spread over filling (for swirled top, see below). Refrigerate until set, about 1 hr. Using a knife, score chocolate layer into 1½-inch squares. 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).
For swirled top:
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.
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. Let stand until set, about 20 min.
Can be made ahead. Store in an airtight container in refrigerator up to 1 week