In a stand mixer with the paddle attachment, beat the butter and sugar until light and fluffy, about 2-3 minutes.
1/2 cup (114g) unsalted butter, 1 cup (200g) granulated sugar
Reduce the mixer to low and add the egg, egg whites, and vanilla extract.
1 large egg, 2 large egg whites, 2 tsp vanilla extract
Stir in half of the flour mixture, mixing until just combined. Add the milk, mixing until just combined. Add the remaining flour and mix again until just combined. Gently fold in the chopped strawberries with a rubber spatula.
1/4 cup (60ml) whole milk, 1 cup (166g) finely chopped strawberries
Divide the batter evenly into the prepared muffin cups. Bake for 20-25 minutes or until a toothpick inserted into the center comes out clean. Let cool completely before frosting.
For the frosting:
Using a stand mixer with paddle attachment, beat the butter and jam until well combined and smooth. Mix in the powdered sugar, one cup at a time. Mix well after last cup. Frost as desired.
3/4 cup 170g unsalted butter, 1/3 cup (100g) seedless pure strawberry jam, 3 cups (375g) powdered sugar
Notes
Leftover cupcakes can be stored in an airtight container in the fridge for up to 2-3 days.