Make the Cake:

1. Preheat oven to 350F and grease and flour two 8″ cake rounds, line with parchment.
2. Remove 1 tablespoon of flour and toss with walnuts in a small bowl. Set aside.
3. In a medium bowl, whisk remaining flour, baking powder, salt and cinnamon until well combined. Set aside.
4. Using a stand mixer fitted with the paddle attachment, beat butter until smooth. Add sugar and beat on med-high until pale and fluffy (2-3 minutes).
5. Reduce speed and add eggs one at a time, fully incorporating after each addition. Add vanilla extract.
6. Alternate adding flour mixture and buttermilk, beginning and ending with flour (3 additions of flour and 2 of milk). Fully incorporating after each addition. Stop mixing when you can still see a bit of flour unincorporated.
7. Fold in chopped walnuts.
8. Divide batter evenly into prepared pans. Smooth the tops with a spatula.
9. Bake for approximately 30 minutes or until a toothpick inserted into the center comes out mostly clean.
10. Place cakes on wire rack to cool for 10 minutes then turn out onto wire rack to cool completely.

Make the Frosting:

1. Place egg whites and sugar into the bowl of a stand mixer*, whisk until combined.
2. Place bowl over a pan of simmering water to create a double-boiler. Whisking constantly, heat the egg mixture until it registers 160 degrees on a candy thermometer. Carefully transfer the bowl onto the stand mixer.
3. Using the whisk attachment, beat the egg white mixture on high speed for 8 to 10 minutes until the bowl is no longer warm to the touch and the meringue is fluffy, glossy and holds a stiff peak.
4. Switch to the paddle attachment. With the mixer on low, slowly add cubed butter and mix until incorporated.
5. Add salt and whip on medium-high until silky and smooth, about 3 to 5 minutes.**

Assemble the Cake:

1. Place one layer of cake onto a cake stand or serving plate. Top with approximately 1 cup of buttercream and spread evenly.
2. Repeat with remaining layer and crumb coat the cake. Refrigerate for 20 minutes.
3. Frost the cake and smooth the sides and top. Use an icing comb on the sides or do a swirl, if desired. Place back in the fridge for 20 minutes to set.
4. Gently press chopped walnuts along the bottom and top edge of cake if desired.
5. Pipe rosettes on the top using an open star tip with the leftover frosting, if desired.