Combine all dry ingredients and whisk together. Add wet ingredients and mix until combined. I use my KitchenAid on low for 2 minutes but you can also do this by hand.
Divide the batter between the pans and bake for approximately 40-50 minutes. Or until a cake tester comes out mostly clean.
Cool in pans on wire rack for 10mins, then turn cakes out onto wire racks to cool completely.
Place frozen raspberries and sugar into a small saucepan and cook over med-high, stirring often, until raspberries are broken down and syrupy (crush with a wooden spoon as you stir).
Beat butter, sugar, and sea salt until pale and fluffy. Gradually add in melted chocolate. Gradually add in cocoa mixture. Whip until well combined.
Let stand 1-2 hours to firm up.
Cut each cake layer in half horizontally. Transfer one layer to serving dish or cake stand. Spread 1/3 of the raspberry sauce over top. Place another cake layer on top and spread approx. 1 cup of the frosting over top. Alternate layers of cake with raspberry sauce and frosting, but leave the very top uncovered. Chill to set for 30mins before frosting outside of cake. Work quickly as the frosting will harden on the cooled cake.
Watch this video to see how to do the rustic swirly frosting.
* You can use store-bought raspberry jam instead. Heat it up a bit if you want it easier to spread. I didn't strain out the seeds, but you can do so if you prefer. Double the recipe for the sauce if you plan to strain seeds out. ** This frosting is best kept at room temperature and served same day. Once the frosting or cake is chilled it becomes very firm and will crack if you try to cut it when it's cold. If you chill the frosted cake, be sure to take it out 1-2 hours before you plan to serve it. The frosting will be a thick fudge consistency. Frosting recipe adapted from Martha Stewart.