Heat the milk in a saucepan and bring to a boil. Remove from heat and cover. In a separate bowl, whisk the egg yolks with sugar, then add flour. Pour a bit of the milk (around 1/3) on the eggs mix. Whisk the eggs with the milk before transferring back into the saucepan with the rest of the milk. Heat everything at medium heat while continuously stirring to thicken the cream. Once it starts boiling, you will have to keep stirring and cooking for 2 more minutes, to make sure the eggs and the flour are thoroughly cooked. Remove from the stove, add the butter, and whisk well. Finally, add the houjicha powder, whisk and use an immersion blender (if you have one) to get a very smooth cream. Transfer in another recipient and protect it with saran wrap directly onto the cream. Let it cool down in the fridge for at least 2 hours before using.