Make the strawberry jam: In a medium saucepan place all the ingredients and bring to a boil. Reduce heat to low and simmer gently, stirring occasionally, until liquid is thick and sticky and drops heavily from the spoon, about 15 minutes. Remove from heat and transfer to a bowl. Allow to cool completely before using (you can put it in the fridge to speed up the process).
Make the blondies: Preheat oven to 350°F/180°C. Line an 8x8-inch pan with parchment paper, leaving enough overhang on the sides for lifting the bars out of the pan after baking.
In a large bowl, place melted butter and brown sugar and whisk until well combined. Whisk in the egg and vanilla extract. Add flour mixture and fold, using a rubber spatula, just until combined. Do not over mix. Fold in chocolate chips/chunks.
Spread dough evenly into prepared pan and smooth the top. Dollop spoonfuls of the strawberry jam over the dough. With a knife or a skewer, gently swirl the two batters together, to get a marble effect. Bake for 30-40 minutes (I prefer fudgy blondies so I bake them for 30-35 minutes). The centre needs to be slightly wobbly when you shake the tin gently. Allow to cool to room temperature on a wire rack, then lift out the blondies of the pan and cut into squares. Serve warm or at room temperature.