Line an 8-inch square pan with parchment, leaving overhang for easy lifting.
Heat-treat the flour: microwave in a shallow bowl in 30-second bursts, stirring, until it reaches 165°F. Cool completely.
Beat butter, brown sugar, and granulated sugar until creamy and slightly fluffy.
Mix in vanilla and milk until smooth.
Stir in the cooled heat-treated flour and salt until a soft dough forms. Add a splash more milk if needed.
Fold in the mini chocolate chips.
Press the dough evenly into the lined pan, smoothing the surface.
Melt topping chocolate chips with oil in short microwave bursts, stirring until silky.
Pour the melted chocolate over the dough and spread into an even layer. Sprinkle with flaky salt if using.
Chill until set, about 1–2 hours. Lift out, slice into bars, and serve.