In a large bowl or measuring cup, whisk together heavy cream, whole milk, vanilla extract, and salt until combined.
In a medium saucepan over medium-high heat, cook 150 g (about 1 cup) sugar and 80 ml (about 1/3 cup) water, gently swirling the pot but not stirring, until the mixture turns a deep golden color, 10 to 15 minutes. Use a pastry brush dipped in water to clean the inner sides of the pan and avoid burning. Working quickly, pour the caramel into an 8" round cake pan. Gently lift and tilt the pan to coat the inside with caramel. Don't worry if the sugar hardens; it will melt down while baking. Place the pan in a large, deep baking dish.
In a large bowl, whisk together eggs, egg yolks, and the remaining 150 g (about 1/2 cup) sugar until combined. Gently mix in the cream mixture until just combined. Pour the mixture over the caramel in the cake pan. Pour enough hot water into the baking dish to come about halfway up the sides of the cake pan.
Bake the flan until just set and the center slightly jiggles, 50 to 55 minutes. Transfer the pan to a wire rack and let it cool. If desired, refrigerate until chilled, at least 2 hours or up to overnight.
When ready to serve, using a small knife or offset spatula, carefully loosen the edges of the flan. If refrigerated, let the flan sit at room temperature before inverting. Invert the pan onto a plate with a shallow rim, gently shake to release the flan, and remove the pan.