In a small saucepan, whisk together sugar and cornstarch until combined.

Add berries and lemon juice and use a spatula to stir until well mixed.

Heat saucepan over medium-low heat and cook, stirring frequently, until the berries soften and begin to release their juices. Use a potato masher or large fork to mash the berries until no large pieces remain.

Continue to cook, stirring constantly, until mixture begins to thicken to a jelly-like consistency.

Once mixture has thickened (this will take several minutes), remove from heat and stir in butter. Transfer to a heatproof bowl, cover, and refrigerate while you make the cookie dough.

Combine butter and sugar in a large mixing bowl and use an electric mixer to beat together until light and fluffy (about 1 minute on high speed).

Add eggs and vanilla extract and stir together until thoroughly combined. Set aside.

In a medium-sized mixing bowl, whisk together flour, cornstarch, baking powder, baking soda, and salt.

Gradually stir dry ingredients into wet until completely combined.

Cover bowl tightly with plastic wrap and chill in the refrigerator for 30-60 minutes. (If dough chills longer than this it can become difficult to manage).

Once the dough has chilled and the filling is completely cooled, remove both from the refrigerator. Scoop ¼ cup (85g) of dough into a ball. Divide the ball into two, and form each half into a shallow nest. Scoop two teaspoons of strawberry filling into one nest and cover with the other. Be sure the filling is completely concealed, you can pinch the edges of the dough together and then very gently roll between your palms to create a round ball.

Place cookie dough balls onto a plate or baking sheet and place in the freezer for 10-15 minutes. Meanwhile, preheat your oven to 375F (190C) and line a baking sheet with parchment paper.

Once dough has chilled and oven is preheated, remove dough balls from freezer and place on prepared baking sheet, spacing cookies at least 2” apart.

Transfer to center rack of 375F (190C) oven and bake for 14-16 minutes, until the edges of the cookies are just beginning to turn a light golden brown. Allow to cool completely on baking sheet before icing (cookies are very fragile when warm!).

Combine sugar, 1 Tablespoon of milk, corn syrup, and vanilla extract in a small bowl and whisk together until combined. Add remaining milk, as needed, until icing is a smooth, spreadable consistency and when you lift a spoon from the icing the ribbon that drizzles back in holds its shape for several seconds before dissolving.

Spread icing evenly over the top of the cooled cookies and sprinkle with nonpareils. Allow frosting to harden before enjoying.