Heat the oven to 350°F (175°C). Line an 8-inch square baking pan with parchment paper, leaving enough overhang on two sides to lift out the cooled blondies.

Toss the finely chopped strawberries and diced rhubarb with the granulated sugar and cornstarch. Let the mixture stand for about 5 minutes, then drain off excess liquid.

Whisk the melted, slightly cooled butter and packed brown sugar together until combined. Add the egg, egg yolk, and vanilla, then whisk until smooth.

In a separate bowl, stir together the flour, baking powder, and salt. Fold the dry mixture into the butter mixture just until no dry streaks remain.

Fold most of the prepared strawberry-rhubarb mixture into the batter, reserving a small amount for the top. Spread the batter in the lined pan and scatter the reserved fruit over the surface.

Bake for 35–40 minutes, until the edges are set and lightly browned and the center is just set. A toothpick should come out with moist crumbs, not wet batter.

Cool completely in the pan for at least 1 hour before lifting and cutting. Chill briefly if you want especially clean, neat slices.