Step 1

Preheat oven to 350°F. Place paper baking cups in 2 (12-cup) standard muffin pans. Prepare cake batter according to package directions; stir in lemon zest and juice, 1 teaspoon of the vanilla, and, if desired, food coloring until combined. Spoon batter into baking cups, filling each about halfway (about 1/4 cup batter in each). Bake both pans at once in preheated oven until a wooden pick inserted in center of cupcakes comes out clean, 19 to 23 minutes, rotating pans halfway through. Let cool in pans 5 minutes. Remove cupcakes from pans to wire racks, and let cool completely, 15 to 20 minutes.

Step 2

Beat butter, strawberry jam, and remaining 3/4 teaspoon vanilla in a medium bowl with an electric mixer on medium speed until light and fluffy, 2 to 3 minutes. With mixer on low, gradually add powdered sugar and salt, beating until smooth, about 1 minute. Transfer frosting to a piping bag, Snip 1 corner of bag to make a 1-inch hole, and pipe frosting onto cooled cupcakes; garnish with strawberry halves.