In a stand mixer with a whisk attachment, beat butter and sugar until light and fluffy.
Add in egg and lemon juice and mix until incorporated.
Slowly add in flour, baking powder, and salt, and continue mixing until fully incorporated.
Turn the mixer off and gently fold in the raspberries. The batter will be thick so the raspberries will break apart.
Pour batter in a 9x9 square baking pan lined with parchment. Bake for 30 minutes or until a toothpick or fork comes out clean. Edges will be slightly golden. Allow to cool completely.
Cream Cheese Frosting
While the blondies are baking, add a small handful of raspberries, about 6-8, 1 tbsp of powdered sugar, and lemon juice to a small sauce pot over medium heat. Cook the raspberries down, mashing them with a fork, about 5 minutes or until the juices begin to simmer and reduce. This should yield about a tablespoon or two of syrup/pulp.
In a stand mixer, whip softened cream cheese and sugar together until light and fluffy. Add in the raspberry syrup/sauce and continue to mix until fully incorporated.
Using an offset spatula, completely cover the cooled blondies with an even layer of frosting. Refrigerate for 10 minutes to allow the frosting to firm up.
Cut into 16 even squares and serve.
Notes
Because the frosting contains dairy, the blondies need to be stored in the refrigerator.