Preheat oven to 300°F. In a large bowl, whisk flour, sugar, cocoa powder, and baking soda. Add the egg, buttermilk, oil, and vanilla; mix until smooth.
Slowly pour in boiling water and stir until fully blended. Divide into two greased 8-inch pans.
Bake for 25–30 minutes, or until a toothpick comes out clean. Let cakes cool and level the tops.
Cream butter and brown sugar in a bowl until smooth. Mix in vanilla, then gradually stir in heat-treated flour.
Add 1–2 tsp milk until it forms a soft dough. Fold in chocolate chips. Roll into small balls and chill in the fridge.
In another bowl, beat cream cheese, brown sugar, milk, and vanilla until smooth. Gently fold in Cool Whip until fluffy. Stir in chocolate chips, then add the chilled cookie dough balls.
Line a springform pan with parchment. Place one cake layer at the bottom. Spread half the ice cream mixture on top and freeze for 30–45 minutes.
Add the second cake layer and spread the remaining ice cream. Freeze the full cake at least 4 hours or overnight until firm.
Remove cake from the pan. Spread Cool Whip around the top and sides. Sprinkle with extra chocolate chips and drizzle with chocolate sauce. Slice with a warm knife for neat pieces and serve immediately.