Step 1

Preheat the oven to 180°C/160℃ fan/350°F/gas mark Grease a 20 cm/8 inch square cake tin and line with baking parchment.

Step 2

Place the butter, sugar, peanut butter, yogurt and vanilla extract in a pan over a low heat and stir until melted and smooth. Remove from the heat.

Step 3

Sift together the flour, cornflour, baking powder, bicarb and salt and add to the wet ingredients. Stir in until no dry lumps remain; the batter will be quite thick.

Step 4

Scrape the batter into the prepared tin and spread level.

Step 5

Heat the jam up slightly until it is runny enough to pipe and transfer it to a disposable piping bag. Snip off the tip and pipe swirls of jam on top of the blondie mixture.

Step 6

Bake for about 25 minutes until a skewer inserted into the centre comes out gunky with moist crumbs but no wet batter. The blondies should be nicely browned but still have a slight jiggle if you shake the tin.

Step 7

Allow the blondies to cool fully in the pan before slicing into 12 pieces.