Preheat your oven to 350°F (175°C) and grease three 6-inch cake pans with butter. Sprinkle with flour and line bottoms with parchment paper.

In a large mixing bowl, whisk together the flour, baking powder, and salt, ensuring no lumps remain. Set aside.

Cream the unsalted butter until smooth and add the granulated sugar. Beat until light and fluffy, about 4-5 minutes.

Add the eggs one at a time, incorporating fully before adding the next. Stir in the vanilla extract.

Alternately mix in the dry ingredients and milk, starting and ending with the flour. Mix gently until just combined.

Divide the batter among the prepared pans and smooth the tops. Bake for 25-30 minutes until a toothpick comes out mostly clean.

Let the cakes cool on a wire rack for about 10 minutes, then turn out onto the rack to cool completely.

Whisk egg whites and sugar over a double boiler until warm. Remove, then whip until stiff peaks form.

Gradually add the butter and cookie butter, whipping until the mixture is smooth and silky.

Layer the cake, spreading frosting between each layer, and adding crunchy cookie butter if desired. Chill for at least 30 minutes.

Apply a final layer of frosting over the entire cake and garnish with crushed speculoos cookies.