Crush cookies into crumbs. Melt 50g butter. Mix into crumbs. Press into 8in springform. Put in freezer. Preheat oven to 160C Make cake batter: Mix dry ingredients. Then add wet ingredients until just combined. Pour into springform. Bake for about 30 minutes until toothpick comes out clean. Make cookie dough frosting. Cream butter, sugar and salt. Add sweetened condensed milk until you get desired consistency. Add chips. Make butterscotch: Melt butter in medium saucepan. Add sugar, ½ cup cream, and salt. Whisk till blended. Leave to boil for 3 minutes. Remove from heat. Stir in vanilla. Once cool add remaining cream and powdered sugar. Once the cake is cool, cut it in half. (Ikea sells a great tool that looks like a giant cheese cutter which works great) Spread the cookie dough frosting on the bottom half. Put the top half back on. Frost with butterscotch buttercream. Decorate as desired.