Place strawberries in a medium bowl. Cover with 1 1/2 tablespoons sugar and refrigerate, 2 hours to overnight.

Preheat the oven to 350 degrees F (175 degrees C). Line 18 muffin cups with paper liners.

Mash strawberry and sugar mixture with a potato masher, or pulse gently in the bowl of a food processor until strawberries are chunky. Strain strawberries over a bowl, reserving juice.

Whisk flour, baking powder, and salt together in a medium bowl.

Beat butter and 1 1/4 cups sugar in the bowl of a stand mixer until pale yellow and fluffy. Add eggs, one at a time, mixing for about 1 minute after each egg and scraping down the sides of the bowl. Mix in vanilla extract and strawberry extract.

Add flour mixture in 2 batches, alternating with the milk, blending well and scraping the sides of the bowl after each addition.

Fold in strawberries and 5 tablespoons strawberry juice with a wooden spoon until well incorporated and you can see bits of strawberry showing throughout the batter. Fill cupcake liners 3/4 full.

Bake in the preheated oven until the tops of the cupcakes are a lovely golden color and they spring back when pressed, about 18 minutes.

Cool in muffin cups for 5 minutes. Remove from tins and finish cooling on a wire rack, about 30 minutes.

While the cupcakes are cooling, combine powdered sugar, butter, milk, lemon zest, lemon juice, and vanilla in the bowl of a stand mixer; whip until creamy.

Pipe frosting onto cupcakes and garnish each with a strawberry slice.