Add the brown sugar to a medium sized mixing bowl. Zest the lemon into the sugar, then use your fingers to rub the zest into the sugar.
Add in the butter. With a hand mixer or a stand mixer fitted with the paddle attachment, beat together the butter and brown sugar until the mixture is light and fluffy (2-3 min.)
Beat in the egg and vanilla until well combined.
Add in the flour, baking powder, and salt. Beat the ingredients together just until no flour streaks remain.
Cover the dough and leave it in the fridge to chill for at least 30 minutes, or for up to 2 days.* While the cookie dough is chilling, make the raspberry curd.
Raspberry Curd
Add the sugar into a small saucepan. Zest the lemon into the sugar, then use your fingers to rub the zest into the sugar.
Add in the frozen raspberries, egg yolks, and salt. Juice the lemon and add 2 tbsp. of lemon juice into the saucepan.
Set the saucepan over medium heat and mix it constantly until it thickens and reaches a temperature of at least 170°F / 76°C. (This took me about 3-5 minutes)
Remove the curd from the heat and pass it through a fine mesh sieve to remove the zest and raspberry seeds.
Immediately whisk in the cubed butter until it melts and the mixture is completely smooth.
Transfer the raspberry curd to the fridge to chill until you're ready to assemble the cookies.
Once you're ready to bake the cookies. preheat the oven to 350°F / 175°C. Line a cookie sheet with parchment paper or with a silicone baking mat.
Use a 3 tbsp. cookie scoop to dish out the cookies onto the prepared cookie sheet. Evenly space the cookies apart to allow room for spreading. Bake for 10-12 minutes, until the sides of the cookies start to turn golden brown.
Immediately after taking the cookies out of the oven, take a bowl that is slightly larger than the cookies and swirl it around over the cookies to make them a little more uniform and circular.
Vanilla Bean Buttercream
In a small bowl, beat together the butter and salt until smooth.
Add in the powdered sugar in two additions, and mix until the mixture is smooth and free of lumps. Then, mix in the vanilla bean paste and heavy cream.
Wait for the cookies to cool down to room temperature before frosting. Transfer the buttercream to a piping bag fitted with your favorite piping tip.
Pipe a tall, large buttercream heart on to the cookies. Fill the center of each heart with raspberry curd, then serve and enjoy!
Notes
*If you're chilling them overnight, you'll want to take the dough out for at least 30 minutes before baking until the dough is warm enough to be scoopable again.Store extra frosted cookies in an airtight container in the fridge.