Heat treat your flour: place in microwave safe bowl and microwave for 45 seconds. Use an instant read thermometer and check the temp is 165. Cool completely.
Once cooled, add salt to flour - mix and set aside
In a large mixing bowl, add butter, peanut butter, brown sugar and granulated sugar and beat on low with a hand mixer until fluffy, about 3 min
Mix in 1 1/2 Tbsp milk and the vanilla.
Add in flour and stir in with a spatula. Add more milk if you need to thin it - about 1 Tbsp at a time. The batter should be pliable and not too stiff.
Using a rubber spatula fold in butterfinger, Reese Pieces, m&ms and pb cups
Store cookie dough in refrigerator (note that it will harden up once chilled because the butter will solidify, you can let it rest at room temperature if desired).