Preheat the oven to 350°F/180°C/Gas Mark 4, no fan (if you have a convection oven, reduce the temperature by about 20° or follow the manufacturer's instructions).
Add soft butter and both types of sugar into the mixing bowl. Mix with an electric mixer or with a stand mixer fitted with a paddle-shaped attachment at medium-low speed until well-combined and fluffy, about 1-2 minutes.
Add eggs and vanilla extract, beat for 1-2 minutes until well-combined.
Sift flour, baking soda, corn starch, and salt into the mixing bowl. Mix at low speed until half-combined.
Add chocolate chips or roughly chopped chocolate (or a mix of both) and sprinkles (if using), mix until combined but don't overmix the dough.
Line the bottom of a 10-inch (25-26 cm) heart-shaped cake pan with parchment paper. Transfer the dough into the pan and even it out with a spatula.
Bake for 30-35 minutes. After 15-20 minutes cover it loosely with a piece of aluminum foil. The cake should be light golden in color, right after taking it out of the oven it can be a little bit jiggly in the middle, but after 1-2 minutes (outside of the oven) it should not be jiggly anymore. The edges of the cake should be completely set and firm to the touch but the center should be soft. A toothpick inserted into the center of the cake should come out with just a few crumbs, it should not come out clean. The center of the cake will collapse while cooling - this is normal. The baking time may vary depending on the oven and cake pan used.
Important: You need to wait for the cake to cool down completely before decorating it with the frosting or before cutting it into servings (it needs to set). This will take about an hour.