Beat butter and sugar in a medium bowl until light and fluffy. Beat in egg and vanilla. Whisk together flour, baking powder, and salt in a separate bowl. Gradually stir the dry ingredients into the butter mixture until a dough forms. Divide the dough evenly among 3 bowls.

Stir almond extract and red food coloring into one portion until evenly colored. Stir melted chocolate into the second portion and chopped walnuts into the third. Line a 9x5-inch loaf pan with waxed paper. Press the almond dough evenly into the bottom of the pan. Top with the walnut dough, then spread the chocolate dough evenly over the top. Cover and refrigerate until firm, about 4 hours.

Preheat oven to 350 degrees F (175 degrees C). Invert the chilled dough onto a cutting board and remove the waxed paper. Cut the dough lengthwise in half, then slice each half crosswise into 1/4-inch-thick slices. Arrange the cookies 1 inch apart on baking sheets.

Bake in the preheated oven until the edges are lightly browned, 10 to 12 minutes. Let cool on the baking sheet for 2 to 3 minutes, then transfer to a wire rack to cool completely.