Preheat the oven to 177C/ 350F. Spray an 8x8" square cake/ brownie pan with baking spray and then line with parchment paper for easier removal.

In the bowl of a stand mixer or a large bowl if using a hand mixer, add the softened butter, sugar and salt. Using the paddle attachment or beaters, mix about a minute until combined. Then add the oil, and beat for about another 2 minutes until a little lighter and fluffier.

Add the eggs one at a time, beating well after each addition, then add the vanilla and beat about 30 seconds more.

Now remove the bowl from the mixer or set the hand mixer aside, and using a big spatula, sift the flour directly into the bowl, folding 1/3 of the flour in alternating with the buttermilk just so that each are almost fully incorporated. Do 3 batches of the flour, with 2 batches of the buttermilk in between. (Folding for this last step as opposed to doing it with the mixer, really ensures a light and fluffy cake.)

Once everything is folded in and you have a smooth batter, pour it into the square cake pan, and bake in the pre-heated oven for about 25 minutes, rotating the pan halfway through. It should spring back to the touch slightly when done and a toothpick insterted into the center should come out with no wet batter. (Begin working on the frosting while the cake is baking.)

Let the cake cool in the pan on a cooling rack for 10 minutes, then use the parchment to lift it out and finish cooling directly on the rack.

While the cake is baking, place the chocolate chips and heavy cream in a medium glass bowl, big enough to sit over a pan of simmering water. Melt the chocolate and cream, carefully, stirring periodically to ensure the sides or bottom don't burn. (Make sure the bottom of the bowl is not directly touching the water.)

Once the chocolate and cream are melted and smooth, remove from the heat and stir in the corn syrup and vanilla. Then set it aside (if your kitchen is cool) or in the fridge to chill and pull out the butter so it can soften.

Once the chocolate mixture is firm and cool, using the whisk attachment of a stand mixer or a handheld mixer whip it until it begins to get fluffy and lighten a bit in color. Add the softened butter and salt and whip until smooth and creamy and no butter specks remain. It should hold somewhat of a peak, but whipping it too much will result in grainy frosting, so watch it closely.

Spread the frosting onto the cooled cake with an offset spatula or spoon and top with sprinkles if using.

The finished cake is best served fresh but can be kept, covered, at room temperature for a couple of days or wrapped in the fridge for a week. (though it prob won't last that long.) Enjoy!