In a small saucepan, combine the berries, juice, zest, vanilla, sugar and a pinch of salt. Cook, covered, over medium-low heat until the berries are soft and begin to burst. Crush as many berries as you can with the back of a wooden spoon or a rubber spatula.

Continue cooking, stirring often, until the berries have broken down completely and you’re left with a sauce-y, slightly thickened mixture. Stir in the corn starch-water mixture and let it come to a boil.

Cook until the mixture has thickened more and a spoon leaves a trail on the bottom of the pan that lasts for a second or two. Remove from heat and set aside to cool.

Grease a 9-inch baking tin and line with parchment. Preheat oven to 350°F.

In a medium saucepan, brown the butter.

Stir the peanut butter into the browned butter and set aside to cool.

In a medium mixing bowl with a hand mixer or in the bowl of a stand mixer, whip the eggs and brown sugar on medium-high speed until pale and fluffy, about 4 minutes. You should feel barely any graininess when you rub the mixture between your fingers

Mix in the vanilla, salt and baking powder, then pour in the brown butter peanut butter mixture and combine. Finally, mix in the flour just until the last streaks of white disappear.

Pour the batter into the prepared tin. Using a piping bag or a baggie with the corner snipped off, pipe several vertical lines of blueberry jam on top of the batter. Run a toothpick or skewer back and forth through the batter, perpendicular to the jam lines, every half-inch or so. Top with sesame seeds, if desired.

Bake for 18 to 20 minutes. Allow to cool in the tin for 5 minutes then use the parchment to lift out the blondies and allow to cool completely on a wire rack.