Line a 20cm/8-inch square baking pan with baking/parchment paper. Place all of the ingredients for the base in a food processor. Blend until combined and to a consistency you like. Pour into the base of the pan. Spread out so it evenly covers the base. Refrigerate until firm.
Drain the macadamia nuts. Place all mint filling ingredients in a high-powered blender and blend until smooth and creamy. You can add more or less mint oil according to your taste. Pour into the base and refrigerate until firm.
Melt the coconut oil in a small saucepan. Stir in the cacao powder, vanilla, and stevia. Mix thoroughly, then pour over the filling. Refrigerate until firm.
To serve, remove from the fridge and cut into 20 squares.
Notes
The slice will soften at room temperature, so keep it refrigerated until serving.