Portuguese for "married," casadinhos are bite-size sandwich cookies that are often served at weddings in Brazil. They're typically filled with dulce de leche, or like in Martha's heart-shaped version, guava.
In a medium bowl, whisk together flour, cornstarch, and salt.
In the bowl of an electric mixer fitted with the paddle attachment, cream together butter and superfine sugar on medium speed until pale and fluffy, about 2 minutes. Add egg, vanilla, and lemon zest and beat until well incorporated. Reduce speed to low and gradually add flour mixture; mix until combined. Divide dough in half; shape each half into a flat disk and wrap in plastic. Refrigerate until firm, about 1 hour.
Preheat oven to 350 degrees. Working with one disk at a time, roll out dough 1/8 inch thick on a lightly floured surface. Using a 1 3/4-inch heart-shaped cutter, cut out cookies. Transfer to parchment-lined baking sheets, spaced 1 inch apart. Refrigerate 20 minutes.
Bake, rotating halfway through, until edges are just golden but still pale on top, 12 to 14 minutes. Let cool on sheets 5 minutes. Transfer to wire racks and let cool completely.
In a small saucepan over medium heat, combine guava paste with 1/3 cup water. Cook until paste has melted completely and mixture is smooth. Remove from heat and let cool completely.
Transfer to a piping bag fitted with a 1/4-inch round tip. Pipe 1 tablespoon filling onto bottoms of half the cookies, then sandwich with remaining cookies. In a small bowl, mix together confectioners' sugar and food coloring until well combined. Sift over cookies. Unfilled cookies can be stored in an airtight container at room temperature up to 3 days; filling can be refrigerated, covered, up to 3 days.