Preheat oven to 200 degrees. In a medium bowl, whisk together flour, polenta, and salt. In a large bowl, using an electric mixer, beat butter and 1/2 cup sugar on medium-high until creamy, 3 minutes. Add egg and 1 teaspoon vanilla and beat until combined. With mixer on low, gradually add flour mixture and beat to combine.
Place 1/4 cup sugar in a small bowl; roll dough into 2-3cm balls and coat with sugar. Transfer to two baking paper-lined baking sheets. With the bottom of a glass, flatten balls to 4cm in diameter. Press a heart-shaped cookie cutter into each (do not cut through). Bake until cookies are golden at edges, 10 minutes, rotating sheets halfway through. Let cookies cool on wire racks.
In a small bowl, whisk together icing sugar, 1/4 teaspoon vanilla, and 1 3/4 teaspoons cold water. Whisk in food coloring. With a small spoon, spread glaze inside heart. Let set 15 minutes.
Unique. Stylish. Beautiful kids stuff. That's what Mathilda's Market is all about. Everything is exquisitely and lovingly handmade, or comes from a small production line. Truly a kids shopping heaven. Mathilda's Markets are held in Sydney, Brisbane, Melbourne, Adelaide and Canberra. www.mathildasmarket.com.au