Preheat your oven to 350°F (175°C). Line an 8×8-inch pan with parchment paper, leaving an overhang for easy blondie removal. Melt the unsalted butter in a medium saucepan over medium heat, swirling occasionally until the butter foams and then turns golden brown with a nutty aroma, about 5 to 7 minutes. Remove from heat and let the butter cool for 10 minutes.

In a medium bowl, combine the all-purpose flour, almond flour, cornstarch, baking powder, kosher salt, and freeze-dried strawberry powder. Whisk thoroughly to evenly distribute and break up any clumps, ensuring a smooth dry mixture.

In a large mixing bowl, whisk the cooled browned butter with the light brown sugar and granulated sugar until smooth. Add eggs one at a time, whisking well after each. Mix in sour cream, vanilla extract, and lemon zest until fully incorporated.

Gradually add the dry ingredient mixture to the wet ingredients, folding gently using a silicone spatula until just combined. Avoid overmixing to maintain tender crumb texture.

Spread half of the batter evenly into the prepared pan. Drop spoonfuls of the seedless strawberry jam over the batter. Spread the remaining batter over the jam layer, then add more dollops of jam on top. Using a knife, gently swirl the jam through the batter. Sprinkle white chocolate chips evenly over the surface.

Bake in the preheated oven for 25 to 30 minutes, or until the edges are set and a toothpick inserted into the center comes out with moist crumbs. Rotate the pan halfway through baking for even cooking.

Allow the blondies to cool completely in the pan on a wire rack for at least 1 hour. Lift them out using the parchment overhang and cut into 16 squares. For cleaner slices, let cool fully; for gooier slices, warm briefly in the microwave for 5 to 10 seconds before serving.