Sift the hojicha powder through a fine strainer into a small bowl. Add a small amount of the milk and blend with a whisk (or matcha brush, if you have one) until smooth and no lumps remain. If it's too paste-like, add a little more milk.
Place the cream and remaining milk in a pot/pan and warm over a medium-low heat until it reaches scalding point - starting to steam a little, and a few bubbles on the edges but not yet simmering.
Meanwhile, whisk together the egg yolks and sugar in a small-medium bowl until well combined and slightly glossy. Becoming glassy is a sign the sugar has dissolved.
Carefully add a small amount (around a tablespoon or two) of the warm cream-milk mixture to the yolk mixture and whisk as you add so it combines without cooking the egg. Once mixed, add a little more, mix then repeat.
After you have added the warm milk around three or four times, pour the eggy mixture in to the main pot/pan and place back on the stove over a low heat. Warm the mixture, stirring constantly, so that it thickens up a little. Take care - DO NOT BOIL. It should warm to around 170F/76C. It should become noticeably a bit thicker but not really thick. Remove from the heat and allow to cool for a couple minutes.
Once it has cooled a couple minutes, add the hojicha mixture and vanilla extract to the custard base and mix in well. At this point, I recommend straining just to make sure you don't have any lumps in there, but you can skip if you like. Let the mixture cool a minute or two more then once cool enough, cover and place in the fridge to chill at least a few hours but ideally overnight.
Once chilled, follow your ice cream maker's instructions to churn the mixture to a soft serve consistency, around 20 minutes. Once soft-serve texture, transfer to a container and place in freezer for a few hours to firm up before serving.