In a medium saucepan, combine the sugar, water, and cream of tartar. Bring to a boil over medium heat. Once boiling, do not stir, as this can cause the sugar to crystallize. Continue cooking until the caramel turns deep amber. Watch closely. It will darken quickly toward the end and can burn in seconds.

Remove from the heat and slowly pour in the heavy cream, whisking constantly. Be careful, as the caramel will bubble vigorously.

Whisk in the sweetened condensed milk, butter, vanilla, and salt until smooth.

Transfer to a heatproof measuring cup or bowl and let cool completely. The caramel will thicken as it cools, so make it the day before or at least 5–6 hours before assembling the cake.

Preheat the oven to 325°F (160°C). Grease the sides of three 8-inch cake pans and line the bottoms with parchment paper.

In the bowl of a stand mixer fitted with the paddle attachment, whisk together the flour, sugar, baking powder, vanilla powder, and salt.

Add the butter and mix on low speed for 2–3 minutes, or until the mixture resembles coarse sand.

In a separate bowl, whisk together the eggs, milk, heavy cream, sour cream, oil, and vanilla until smooth.

Add half the wet ingredients and mix on low speed until the flour is moistened and the batter forms a smooth, thick paste. Scrape down the sides and bottom of the bowl. Add the remaining wet ingredients and mix on low speed until smooth and combined, scraping the bowl once more as needed.

Divide the batter evenly among the prepared pans, about 450 g per pan. Bake for 25–30 minutes, or until a toothpick inserted into the centers comes out with a few moist crumbs.

Let the cakes cool in their pans on a wire rack for 20–30 minutes. Remove from the pans and let cool completely on the racks before frosting.

In the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium speed for 2–3 minutes, or until light and fluffy.

Add the powdered sugar and mix on low speed until incorporated. Increase to medium speed and beat for 4–5 minutes, or until light and fluffy.

Add the salted caramel, heavy cream, and vanilla. Mix until smooth and combined.

Place the first cake layer on a serving platter or cake stand. The layers should bake fairly flat, so trimming may not be necessary.

Spread a thin layer of buttercream over the cake, then pipe a thick border of frosting around the outer edge. Spread about ⅓ cup of the cooled caramel inside the border, keeping it contained within the frosting.

Place the second cake layer on top and repeat with the frosting border and caramel. Place the final cake layer upside down for a flat top.

Spread a thin layer of buttercream over the entire cake to create a crumb coat. Refrigerate for 20–30 minutes, or until the frosting is firm.

Frost the top and sides with the remaining buttercream, reserving any extra for decorating if desired.

To create the caramel-swirled sides, spread small amounts of caramel randomly over the frosted sides. Run a cake scraper or offset spatula around the cake to gently swirl the caramel into the buttercream.

Spread additional caramel over the top, if desired, and finish with flaky sea salt.