Preheat your oven to 350°F (175°C). Line an 8x8 inch baking pan with parchment paper, leaving an overhang on two sides.
Melt the butter and dark chocolate together in a large microwave-safe bowl in 30 second bursts, stirring each time. Whisk in both sugars, cool for 5 minutes, then whisk in the yogurt and vanilla until glossy.
In a separate bowl, whisk the flour, cocoa powder, baking soda and salt. Fold into the chocolate mixture just until combined and spread evenly into the pan.
Microwave the raspberry jam for 10 to 15 seconds to loosen it. Drop small spoonfuls over the batter and drag a toothpick through in loops to create swirls.
Scatter the fresh raspberries over the top and press them gently halfway into the batter.
Bake for 28 to 32 minutes, until the edges are set and a toothpick inserted in the center comes out with moist crumbs.
Let the brownies cool completely in the pan, about 2 hours, then chill for 30 minutes for the cleanest slices. Cut into 16 squares.