Pre-heat oven to 350 degrees and line 2 baking sheets with parchment paper.

Beat the butter and 1 cup of granulated sugar till light and fluffy. Scrape down the sides. Add the egg, vanilla extract and seeds from the vanilla bean and beat again. Scrape down the sides. Add the flour, baking powder and salt and beat again till well combined. Remove half the dough.

Add the green food coloring to the dough in the mixer and beat till the color is incorporated, adding more if needed till the desired shade is reached. You want the color to be vibrant, but light, so that it contrasts with the blue when the cookies are baked. Repeat with the reserved dough and food coloring.

Place 1/4 - 1/2 cup of granulated sugar into a small bowl.

Working with one piece of dough in each color, divide each into 2 - 3 small pieces, then gather them together and gently roll into a ball. Don't overwork so that the colors stay distinct. Roll in the granulated sugar and place on the other baking sheet, making sure that the prettiest side is facing up. Repeat with the remaining dough, for a total of 24 - 28 cookies. Bake for 8 minutes, or till just turning crinkly on top.

Remove the baking sheets from the oven and place on wire racks. Working quickly, place one candy heart on top of each cookie, pressing gently to adhere. Let the cookies cool on the tray for 5 more minutes, then transfer to the wire racks to cool completely.