Mash avocado in a large bowl and stir in lime juice, zest and buttermilk. Whisk until well blended and the consistency of heavy cream.
Combine flour and next 4 ingredients.
Either by hand or with a food processor, cut in butter until well blended, leaving a few larger pieces of butter (about the size of lentils).
Dump the flour mixture into the avocado mixture and stir until just combined. Pour onto a work surface and knead just until dough comes together. Give it 3 final turns and pat out into an 8 inch circle.
Combine remaining sugar, chili powder, and salt. Brush dough with buttermilk and sprinkle on the sugar mixture. Cut into 8 wedges and space evenly on a baking sheet.
Bake for about 17 minutes, until bottoms are medium brown. It is important to allow these to cool completely before eating.