Preheat oven to 350F. Grease and flour three 6" (or two 8") cake rounds and line with parchment.
In a medium bowl, whisk flour, baking powder, and salt until well combined. Set aside.
In a measuring cup, combine maple syrup and maple liqueur (or milk).
Using a stand mixer fitted with a paddle attachment, beat butter and sugar on med-high until pale and fluffy (approx 3mins). Reduce speed and add eggs one at a time fully incorporating after each addition.* Add vanilla.
Alternate adding flour mixture and maple mixture, beginning and ending with flour (3 additions of flour and 2 of maple). Fully incorporating after each addition.
Bake for 30-35mins or until a toothpick inserted into the center comes out mostly clean.
Place cakes on wire rack to cool for 10mins then turn out onto wire rack.
Preheat oven to 350F.
Whisk flour and maple sugar in a medium bowl. Add enough melted butter so that the mixture starts to clump. Spread on a parchment lined baking sheet and bake for approx. 8mins. Cool completely. Break streusel apart if needed. **
Maple Swiss Meringue Buttercream:
Place egg whites and maple sugar into the bowl of a stand mixer, whisk until combined.***
Place bowl over a double boiler on the stove and whisk constantly until the mixture is hot and no longer grainy to the touch (approx. 3mins). Or registers 160F on a candy thermometer.
Place bowl on your stand mixer and whisk on med-high until the meringue is stiff and cooled (the bowl is no longer warm to the touch (approx. 5-10mins)). The meringue may not have the shine of a typical meringue.
Switch to paddle attachment. Slowly add cubed butter and mix until smooth.****
Add maple liqueur (or maple syrup) and whip until smooth.
Place one layer of cake on a cake stand or serving plate. Drizzle with 2 tsps maple syrup if desired. Top with approximately 2/3 cup of buttercream. Spread evenly. Top with approx 1/3 cup of maple streusel. Press gently into buttercream.
Repeat with next layer and place the final layer on top of the cake. Crumb coat the cake and chill in fridge for 20mins.
Generously frost and smooth the outside of the cake. Using an icing comb to do the design on the sides and a 1M tip to do a rope border on top. Fill the middle with extra streusel if desired.
* The eggs will take longer than normal to incorporate, be sure to whip well with each addition. Add a bit of the flour mixture after the last egg if needed to help it come together. ** This recipe makes much more than you'll need, but I like to have the option to pick out the larger chunks. Feel free to halve the recipe if you prefer. *** Wipe your mixer bowl and whisk down with lemon juice or vinegar to make sure it is completely grease free and make sure there is no trace of yolk in your whites or your meringue will not stiffen. **** The buttercream may look like it's curdled at some point. Keep mixing until it is completely smooth. If it looks soupy, place it in the fridge for 20mins and rewhip.