Preheat oven to 175°C (350°F). Line an 8x8-inch (20x20 cm) square baking pan with parchment paper leaving an overhang for easy removal; lightly butter the parchment.

Melt butter and white chocolate

In a heatproof bowl set over a pan of barely simmering water (double boiler) or in short bursts in the microwave (stirring every 20–30 seconds), melt the 170 g butter with 200 g chopped white chocolate until smooth. Remove from heat and let cool for 3–5 minutes.

Combine sugar and eggs

Whisk the 300 g granulated sugar into the slightly cooled chocolate-butter mixture until incorporated. Add the 3 large eggs one at a time, whisking thoroughly after each addition to create a glossy batter. Whisk in 2 tsp vanilla extract.

Add dry ingredients

Sift together 140 g all-purpose flour and 1 tsp salt. Fold the dry ingredients into the wet batter gently until just combined — do not overmix. Fold in 60 g sour cream (or Greek yogurt) to add moisture and tenderness. If using, fold in 60 g white chocolate chunks.

Bake

Pour batter into the prepared pan, smoothing the top. Bake in the preheated 175°C oven for 22–28 minutes, or until the edges are set and a toothpick inserted into the center comes out with a few moist crumbs (for fudgy brownies). Allow to cool in the pan on a wire rack for 30 minutes, then lift out using the parchment overhang and cool to room temperature before slicing.

Chocolate Sauce

Make the sauce while brownies cool. Chop 100 g semi-sweet or dark chocolate and place in a heatproof bowl. Heat 120 ml heavy cream in a small saucepan until it just begins to simmer (do not boil). Pour hot cream over the chopped chocolate and let sit for 1 minute, then stir until smooth. Stir in 15 g butter, 1 tbs corn syrup or honey (if using) and a pinch of salt until glossy. Keep warm or let cool to a pourable warm temperature.

Whipped Cream & Assembly

Whip the cream

Chill a medium bowl and whisk (or bowl of a stand mixer) for 10 minutes if possible. Pour 240 ml cold heavy cream, add 2 tbs powdered sugar and 1 tsp vanilla extract. Whip with a handheld mixer or stand mixer on medium-high speed until soft peaks form (do not overwhip). Keep chilled until assembly.

Slice and plate

Cut the cooled brownie into 4 equal squares (servings). Place one brownie square on each dessert plate.

Add ice cream and sauce

Place one generous scoop of french vanilla ice cream next to or on top of each warm (or room-temperature) brownie square. Spoon warm chocolate sauce over the brownie and a little over the ice cream for contrast (about 2–3 tbs sauce per serving).

Finish with whipped cream & garnish

Top each plated dessert with a generous dollop of fresh whipped cream (about 2–3 tbs). Optionally sprinkle a pinch of flaky sea salt and add a mint leaf or chocolate shavings for presentation.

Serve

Serve immediately so the ice cream contrasts with the warm brownie and sauce. Leftover brownies keep in an airtight container for up to 3 days; rewarm briefly before serving.