Create a highly detailed black and white vector pencil sketch illustration featuring two adorable squirrels peering out from a tree hollow. The artwork must be rendered entirely in monochrome using only black ink on white background with no color whatsoever. Employ intricate cross-hatching and fine line work to create realistic fur textures on the squirrels. The tree bark should show detailed wood grain patterns with cracks and natural irregularities created through careful shading techniques. Focus on capturing the expressive, bright eyes of the squirrels with precise highlights. The composition should be centered with the hollow creating a natural frame around the subjects. Use varying line weights and densities to achieve depth and three-dimensionality. The overall style should mimic traditional graphite pencil drawing with professional engraving quality.