Preheat oven to 350F. Grease and flour three 6" cake rounds and line with parchment.
In a medium bowl, whisk flour, baking powder, and salt until well combined. Set aside.
Using a stand mixer fitted with a paddle attachment, cream butter and sugars on med-hig 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 milk, beginning and ending with flour (3 additions of flour and 2 of milk). Fully incorporating after each addition.
Toss chocolate chips in 1 Tbsp flour. Gently fold in chocolate chips and mix until just incorporated.
Bake for 30-35 mins 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 and cool completely.
Oatmeal Cookie Crumble (optional):
Preheat oven to 350F. Whisk flour, oats, and brown sugar in a medium bowl. Add enough melted butter so that the mixture starts to clump. Spread onto a parchment lined baking sheet and bake for 8-10 mins. Cool completely. Break crumble apart if needed.
This is optional, if you prefer you can just use crushed cookies instead.
Oatmeal Cookie Buttercream:
Place egg whites and sugar into the bowl of a stand mixer, whisk until combined.*
Place bowl over a pot with 1-2" of simmering water and stir constantly with a whisk until the mixture is hot and no longer grainy to the touch or reads 160F on a candy thermometer (approx. 3mins).
Place bowl on your stand mixer and whip on med-high until the meringue is stiff and cooled (the bowl is no longer warm to the touch (approx. 5-10mins)).
Switch to paddle attachment. Slowly add cubed butter and mix until smooth.**
Add vanilla and cookie crumbs whip until smooth.***
Assembly:
Place one layer of cake onto a cake stand or serving plate. Top with 2/3 cup of frosting and spread evenly. Sprinkle with about 1/4 - 1/2 cup crumble (smaller pieces). Sprinkle mini chocolate chips on top if desired. Press gently into frosting. Repeat with next layer. Crumb coat the cake. Chill for 20mins.
Frost the cake and smooth the sides and top. Do a rope border using a Wilton 6B piping tip. Place crushed cookies along the bottom of the cake if desired.
Notes
* 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. See this post for tips.*** You can use less cookie crumbs if you like. Add them 1/4 cup at a time to taste.