You can do it with the sculpt tools:
Create a cube, stretch it, subdivide and bevel its edges:
To get a topology that is high enough for sculpting you can either subdivide with a Subdivision Surface modifier and apply, or enable the sculpt Dyntopo option, or use the Multiresolution modifier. With the latter solution, click several times on the Subdivide button, put all the levels at the same value:
Switch to Sculpt mode, disable the Display Fast Navigate option to avoid the glitches when you'll rotate your view:
Begin to sculpt, use mainly the Draw brush I guess, left click to create bumps, Ctrl left click to dig, Shift left click to smooth. With this solution the sculpting is only virtual, you can see your intial shape if you disable the modifier visibility:
Give your object a material with chocolate color and lower the Roughness a bit down. You can add a bit of fake bump, and also create a b&w mask if you want to create some hazelnut sctraches, etc:
Now you can either apply the modifier and you'll have real topology, or keep it and bake the bumps, in that case you'll need to activate the Bake from Multires option in the Bake panel, choose Bake Type > Normal, and also put the modifier's Level Viewport at 0 before the bake.