Chocolate Hazelnut Slices

In bowl, whisk boiling water into cocoa until smooth; let cool to room temperature.
In large bowl, beat butter with sugar until fluffy; beat in egg.
Beat in cocoa mixture; stir in vanilla.
Whisk flour with baking powder; stir into butter mixture in 2 additions.
Stir in chocolate chips and hazelnuts.
Divide dough into quarters; form each into 8 inch long log.
Wrap tightly in waxed or parchment paper; press on work surface to flatten bottom.
Turn and flatten 2 more times to form triangular log.
Refrigerate until chilled, about 2 hours.
(Make ahead: Freeze in airtight container for up to 1 month; do not thaw to continue.)
Unwrap each log and brush with water.
Spread sprinkles on flat surface; press log into sprinkles to coat completely.
Cut into 1/4 inch thick slices.
Place, 2 inches apart, on parchment paper lined or greased rimless baking sheets.
Bake in top and bottom thirds of 375 F. oven until slightly puffed and set, 12 to 14 minutes, rotating and switching pans halfway through.
Let cool on pan on rack for 5 minutes.
Transfer to rack; let cool completely.
(Make ahead: Store in airtight container for up to 1 week or freeze in layers separated by waxed paper for up to 1 month.
Makes about 8 dozen.
TIP: Hazelnuts are also known as filberts.
If you can find only whole hazelnuts, toast them in 350 F. oven for 10 minutes.
Wrap in tea towel and rub vigorously through towel to slough off most of the dark brown skin.
Let cool and then chop.
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