1. Preheat oven to 400°F. Line 2 large baking sheets with parchment paper.
2. Make cookies: Sift flour, baking soda and salt. In another bowl, beat butter, sugar, zest and vanilla until fluffy. Beat in yolk. Add flour mixture in 2 batches, alternating with lemon juice.
3. Form balls using 2 Tbsp. dough for each. Arrange 2 inches apart on sheet; flatten each to a 3-inch round. Bake, 1 sheet at a time, for 8 minutes, until edges are golden. Let cool on a rack for 5 minutes. Cut each cookie into 4 wedges; cool completely.
4. Decorate cookies: Melt chocolate in a bowl set over a pan of simmering water. Spread each cookie with chocolate, add 2 pretzel horns, 2 jelly bean eyes and an M&M nose to each. Dot each eye with chocolate. Let stand until chocolate is set, 10 minutes.