Finely dice strawberries and mix with 1 teaspoon of sugar, set aside.
In a small heavy bottomed pot, add heavy cream, milk, and salt and cook on medium heat until it begins to gently simmer, at this point turn off the heat.
In a separate bowl, beat egg yolks and sugar until they are pale.
In a small bowl, whisk together corn starch and water. Pour cornstarch paste into the custard mixture.
Return the custard to the saucepan and cook over medium low heat, stirring constantly, until the custard is thickened and glossy, about 3-5 minutes.
Remove from heat and add butter and vanilla; stir until smooth.
Stir in diced strawberries, transfer to a bowl and cover with plastic wrap and refrigerate custard until cold and ready to use. This can be made up to 2 days in advance.
In a small bowl, combine warm milk and yeast. Allow to sit until yeast begins to "bloom".
Once yeast mixture is foamy, stir in melted butter and sugar.
Whisk in eggs.
Begin with adding 3 ½ cups of flour and the salt. Knead mixture until dough forms and ball and no longer sticks to the sides of the bowl. If needed, add more flour in 1 tablespoon increments at a time.
Once dough is smooth, place in a large greased bowl, cover and let rest until dough has doubled in size, at least 2 hours.
Once dough has risen, place on a well floured surface and roll a flat disk, about 1-2 inches thick.
Place on a wax paper lined cookie sheet and cover lightly. Allow dough to rest for 30-45 minutes.
Once donuts have cooled slightly, coat in sugar. Set aside to cool completely.
Depending on the altitude and/or humidity levels, you may need more flour.
Doughnuts are best enjoyed the same day.
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