First, make the pink batter. Preheat the oven to 175 C. Line a 6x4 loaf tin (see notes) with non-stick baking paper, leaving a little overhang for easy removal and set aside.
In a mixing bowl, beat the butter and sugar till pale and fluffy, using a hand mixer on medium speed for best results. Add the lemon zest, egg and vanilla, beat to combine.
Sift in the flour, baking powder and salt together into the bowl. Fold to combine, then add the food colour and lightly beat to combine. The batter is thick and should remain that way but if you find it excessively thick, add a tiny splash of milk. Spread into the prepared tin and smoothen the top.
Bake for 25 to 30 minutes or until a toothpick poked in the center comes out clean. Set aside to cool in the tin for about 15 minutes, then lift using the overhang and place on a wire rack to cool for about 20 minutes.
Place the cake in the fridge for 20 minutes, then slice off any uneven bits on top. Now slice it in half horizontally. Use a 2 inch wide heart-shaped cookie cutter to cut out the hearts. I got 5 heart shapes but a smaller cutter will yield more. Place the hearts on a tray and freeze for 30 minutes.
Rinse and thoroughly dry your mixing bowl and beaters, along with any spoons or spatulas you plan to reuse. Make sure there is no pink colour left on anything. Line the same loaf tine with non-stick baking paper again (or reuse the earlier one making sure you've scraped off any pink crumbs).
Now make the plain batter. Follow the same method as the pink batter, except that this time there's no food colour.
Get the frozen pink hearts out and spread a thick layer of the plain batter on the bottom and sides of the loaf tin, making sure you have enough reserved for the top. Arrange the hearts close together in one straight line, in the center of the tin. Spoon the remaining batter around the edges and on the top, making sure the hearts are completely covered (see photos above).
Bake for 40 to 45 minutes, until a toothpick poked in the center or around the sides comes out clean, but start checking for doneness at 30 minutes. If the top is browning too fast, cover it loosely with foil.
When the cake is done, allow it cool in the tin for 15 minutes, then remove it using the paper overhang and place on a wire rack to cool completely.
To make the glaze, combine the lemon juice and sugar in a small bowl. Stir thoroughly to combine, then add the food colour and stir again. You will have a thick glaze which needs to be poured immediately onto the slightly warm cake, so that it sticks and sets.
Use any scraps from the pink as decoration, by crumbling them up and sprinkling them onto the glaze. Let the cake cool completely before slicing and be ready to be surprised! Happy Valentine's week :)