In a medium saucepan, combine the frozen strawberries, granulated sugar, and pinch of kosher salt. Cook over medium heat, stirring occasionally, until the strawberries release their juices and soften, approximately 8–12 minutes.

Use a fork or spatula to break down the berries, leaving a few small pieces for texture.

Continue simmering over medium-low heat, stirring frequently, until the mixture is thick, concentrated, and jam-like. Allow approximately 15–25 additional minutes, depending on the strawberries and the width of your saucepan. The filling should mound slightly on a spoon rather than run off like syrup.

Transfer the filling to a shallow bowl, cool briefly, then cover and refrigerate until completely cold and thickened, approximately 1–2 hours. If it remains too loose after chilling, return it to the saucepan and reduce further.

Preheat to 325°F. Grease three 9-inch round cake pans, line the bottoms with parchment paper, and lightly grease the parchment.

In a medium bowl, whisk together the cake flour, black cocoa powder, baking powder, baking soda, kosher salt, and espresso powder.

In a the bowl of a stand mixer, beat together the butter, oil, granulated sugar, and dark brown sugar until combined well.

Add the eggs one at a time, mixing together in between. Scrape down the sides of the bowl in between each addition.

Add in the sour cream, buttermilk, vinegar, and vanilla and beat until combined well.

Gradually add the dry ingredients to the wet ingredients, mixing gently until just combined. The batter should be relatively loose and pourable.

Distribute evenly among the three prepared cake pans.

Bake for approximately 22–28 minutes, or until the centers spring back lightly when touched and a toothpick inserted into the center comes out with a few moist crumbs. Begin checking around 20 minutes.

Let the cake layers cool in their pans for 10–15 minutes, then carefully transfer to wire racks to cool completely. For easier assembly, refrigerate the cooled layers for approximately 30 minutes.

In a stand mixer fitted with the paddle attachment, beat the softened butter on medium speed for 3–4 minutes until creamy and fluffy.

Add the sifted black cocoa powder and beat until smooth and fully incorporated.

Gradually mix in the sifted powdered sugar, about 1/2 cup at a time, on low speed.

Mix in the vanilla, kosher salt, optional espresso powder, and 3 tablespoons heavy whipping cream.

Beat on medium speed for 2–3 minutes until creamy and fluffy. Add additional heavy cream, 1 tablespoon at a time, until the frosting reaches your desired consistency. I generally use 6 tbsp for this recipe.

Place one completely cooled cake layer on a cake stand or serving plate. Level the top if necessary.

Transfer some buttercream to a piping bag and pipe a thick ring around the outer edge of the cake layer.

Spread about half of the chilled strawberry filling inside the buttercream dam.

Place the second cake layer on top, gently press down, and then repeat with another frosting dam and layer of jam. Finish with the third cake layer, flat-side up.

Spread a thin layer of buttercream over the top and sides of the cake. Refrigerate for approximately 20–30 minutes until firm (this will help to frost smoothly with the remaining buttercream).

Apply the remaining buttercream to the top and sides, smoothing with an offset spatula or creating decorative swirls.

Garnish with fresh strawberries, chocolate shavings, or crushed freeze-dried strawberries, if desired.

Refrigerate for at least 30–60 minutes to help the cake set. Let sit at room temperature for approximately 20–30 minutes before slicing.