To heat treat flour: Preheat the oven to 350°F and line a baking sheet with parchment paper or a silicone baking mat. Spread flour onto baking sheet and bake for 5 minutes. Allow to cool completely before using.
Line two 8 x 8-inch square baking pans with parchment paper, leaving a little overhang on all sides of the pans.
In a stand mixer fitted with the paddle attachment, beat butter, brown sugar, and granulated sugar on medium high for about 3 minutes, or until light and fluffy.
Add vanilla and milk into the butter and sugar mixture and beat until incorporated.
Add heat-treated flour and salt to the bowl and beat on low until just combined.
Using a spatula, fold in the chocolate chips, chocolate candies, and rainbow sprinkles.
Press half of the dough into each prepared baking pan, smoothing out as much as possible with your hands or with the base of a measuring cup. Freeze for 2 hours or overnight.
Soften the ice cream at room temperature until it is a spreadable consistency, about 15-20 minutes. Pull out both pans of dough from the freezer and remove dough from the first pan using the parchment paper overhanging on the sides.
Spread the softened ice cream over the dough in the second pan as evenly as possible. Take the dough from the first pan and place it on top of the ice cream you’ve spread, then press down to smooth and remove the parchment. Freeze for at least 5 hours or overnight.
Once the sandwiches have been frozen, remove from the pan using the remaining parchment hanging over on the sides.
To serve, run a knife under hot water and cut into nine even squares. Let stand for 5 minutes to soften, then enjoy!