Cream butter and sugars with a hand mixer or stand mixer.
Add eggs one at a time, beating well after each addition.
Stir in vanilla, salt, and baking powder.
Add flour ¼ cup at a time. Incorporate all before adding the next ¼ cup, and continue until all of the flour is well-combined into the batter.
Line a square baking dish with parchment paper. Transfer half of the blondie batter to the baking dish. Spread the strawberry jam in a relatively even layer on top of the blondie batter. Top with the remaining batter.
Bake for about 30-40 minutes, or until the top of the blondies are light brown.