For Cake:
1. Grease and line three, 8-inch round baking pans. 

2. Preheat oven to 325°F. 

3. In a large mixing bowl, sift together dry flour, baking powder, and sugar ingredients. 

4. Using a paddle attachment, add the softened butter to the bowl of dry ingredients and mix on low speed until the mixture looks like coarse sand, approximately 3 minutes. The butter should no longer be clumpy. 

5. Mix the wet ingredients in a separate bowl: eggs, milk, sour cream, vanilla, and maple emulsion. 

6. Add the wet and dry ingredients to the sandy flour mixture in three parts, mixing on medium speed until everything is combined. 

7. Scrape down the bowl to ensure everything is incorporated.

8. Pour batter into your three, 6-inch greased cake tins. 

9. Bake cakes in the oven for approximately 30-35 minutes or until a skewer inserted in the middle comes out clean. 

10. Carefully remove the cakes and set them on a wire rack to cool completely. Wrap in plastic wrap and place on the counter to cool.

For Swiss Meringue Buttercream:
1. Wipe down the mixing bowl, whisk attachments, and all other equipment with white vinegar to ensure no fat residue on anything before starting. 

2. Add the egg whites and sugar to a stand mixer bowl set over a double boiler. 

3. Stir constantly until the egg whites reach 150°F-160°F. 

4. Remove from the heat and let sit for 10 minutes. 

5. Whip the egg white sugar mixture on high-speed using the whisk attachment until medium-stiff peaks form. 

6. Turn the mixer down to the lowest speed and slowly add the softened butter in chunks.

7. Add vanilla and maple emulsion and whip on high speed for 8 minutes. 

8. Place the bowl back over your double boiler, about 30-60 seconds, until small puddles form at the bottom of the bowl. This is called tempering. 

9. Transition to the paddle attachment and beat for five more minutes on low speed until it is shiny and completely smooth. 

10. DOUBLE THIS RECIPE IF YOU WANT EXTRA BUTTERCREAM FOR CAKE DECORATING