Start by placing the raspberry jam in a microwave-safe bowl, heating it in 15-second intervals until warmed through. Stir in high-quality white chocolate until completely smooth and glossy. Pour the mixture into small silicone molds or an ice cube tray and freeze for at least an hour, ensuring the filling is firm and ready for the molten center of your lava cake.

In a medium saucepan over low heat, melt the butter along with the remaining white chocolate, stirring gently until smooth. Remove from heat and whisk in granulated sugar until fully combined. Beat in eggs one by one, ensuring each is well integrated before adding the next. Gently fold in cornstarch and flour until just combined, creating a velvety batter for your cake.

Preheat your oven to 350°F (180°C) and butter four ramekins generously to prevent sticking. Dust the insides with cornstarch and tap out any excess.

Spoon the batter into the prepared ramekins, filling them about three-quarters full. Carefully place a frozen raspberry filling in the center of each ramekin, then top with more batter until they are nearly full.

Transfer the assembled ramekins to the preheated oven and bake for 15-18 minutes. The edges should be set but the centers remain soft with a slight jiggle.

Once baked, allow the cakes to cool for a minute before carefully inverting them onto serving plates. Garnish with fresh raspberries and serve immediately.