Combine the dry ingredients in a bowl: all-purpose flour, baking, baking soda, cornstarch, and salt.
180 gs all-purpose flour, 1/2 tsp baking powder, 1/2 tsp baking soda, 1 tsp cornstarch, 1/2 tsp salt
In a stand mixer (or a bowl with an electric mixer), add the butter and sugar and whisk together on high speed for 3-5 minutes until well combined.
115 gs unsalted butter, 200 gs white granulated sugar
Add the room temperature egg* and vanilla essence into the butter mixture and whisk together until well combined. Make sure the eggs are fully incorporated into the butter and there is no liquid left in the batter.
1 egg, 2 tsp vanilla essence
With the mixer on slow, add the dry ingredients into the batter and keep mixing on low speed until everything is well incorporated and there are no streaks of flour still remaining in the mix. You will have a nice, thick dough at this point.
Wrap the prepared cookie dough tightly in cling film and let it chill in the fridge for 8 hours, or ideally overnight.
Prepare the jam filling: Transfer the jam into a piping bag and pipe (or spoon) small dollops on a tray and let them chill in the freezer for 1 hour until completely set.
100 gs strawberry jam
Fill the cookie dough: Take the chilled cookie dough out of the fridge and form the cookie dough balls. I weigh mine at around 60 gs each. Flatten the cookie dough balls into a disk. Now, place the jam filling in the center of the cookie dough disks. Then, fold the cookie dough from all four sides around the filling and roll it into a ball using your hands. Make sure the filling is covered with dough on all sides. Form rough circles, then let the cookie dough chill in the fridge for 10-15 minutes.
Add the sugar for the topping in a bowl and roll the chilled cookie dough calls in the mixture.
100 gs white granulated sugar
Bake the cookies. Preheat the oven to 170 degrees Celsius and cover a baking tray with baking paper. Place the cookie dough balls on the tray (leaving enough space for each cookie to spread) and bake the cookies for 10-15 minutes, until the edges are set but the center is still soft. These cookies spread a lot so make sure you leave enough space around each cookie.
As soon as the cookies come out of the oven, shape them using a circular object like a jar lid, a mug, or a cookie cutter. Let the cookies cool before digging in.