Spray a 9" springform pan with non-stick baking spray.
Prepare the cake batter first. In a stand mixer fitted with a paddle attachment, cream together butter and sugar. Add in eggs, then dry ingredients. Add in milk last. Scrape down sides as needed. Pour batter into prepared pan and set aside.
Peel, core and slice apples. Place apples in an even layer over the cake batter.
To make the streusel, place all ingredients in a food processor. Process until it resembles bread crumbs. Cover the apples with the streusel.
Bake cake for 1 hour, or until the top looks toasted. Let it cool for 10 minutes in pan, then run a knife around the pan and unmold the cake pan.
To make the glaze, sift powdered sugar in a medium sized bowl. Add in baileys Irish cream, 1 Tbsp at a time until you reach desired consistency.
Add in a pinch of salt, then transfer the glaze to a piping bag.
Cut slices of cake, and drizzle on the glaze as desired.